diff --git a/oauth2/oauth2-resource-server/src/main/java/org/springframework/security/oauth2/server/resource/introspection/RestClientSpringOpaqueTokenIntrospector.java b/oauth2/oauth2-resource-server/src/main/java/org/springframework/security/oauth2/server/resource/introspection/RestClientSpringOpaqueTokenIntrospector.java
new file mode 100644
index 0000000000..a3fe718dc0
--- /dev/null
+++ b/oauth2/oauth2-resource-server/src/main/java/org/springframework/security/oauth2/server/resource/introspection/RestClientSpringOpaqueTokenIntrospector.java
@@ -0,0 +1,355 @@
+/*
+ * Copyright 2004-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.security.oauth2.server.resource.introspection;
+
+import java.io.Serial;
+import java.net.URI;
+import java.net.URLEncoder;
+import java.nio.charset.StandardCharsets;
+import java.time.Instant;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+
+import org.springframework.core.ParameterizedTypeReference;
+import org.springframework.core.convert.converter.Converter;
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpMethod;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.MediaType;
+import org.springframework.http.RequestEntity;
+import org.springframework.http.ResponseEntity;
+import org.springframework.security.core.GrantedAuthority;
+import org.springframework.security.core.authority.SimpleGrantedAuthority;
+import org.springframework.security.oauth2.core.OAuth2AuthenticatedPrincipal;
+import org.springframework.security.oauth2.core.OAuth2TokenIntrospectionClaimAccessor;
+import org.springframework.security.oauth2.core.OAuth2TokenIntrospectionClaimNames;
+import org.springframework.util.Assert;
+import org.springframework.util.LinkedMultiValueMap;
+import org.springframework.util.MultiValueMap;
+import org.springframework.web.client.RestClient;
+
+/**
+ * A Spring implementation of {@link OpaqueTokenIntrospector} that verifies and
+ * introspects a token using the configured
+ * <a href="https://tools.ietf.org/html/rfc7662" target="_blank">OAuth 2.0 Introspection
+ * Endpoint</a>, using {@link RestClient} for HTTP communication.
+ *
+ * @author Andrey Litvitski
+ * @since 7.1
+ */
+public class RestClientSpringOpaqueTokenIntrospector implements OpaqueTokenIntrospector {
+
+	private static final String AUTHORITY_PREFIX = "SCOPE_";
+
+	private static final ParameterizedTypeReference<Map<String, Object>> STRING_OBJECT_MAP = new ParameterizedTypeReference<>() {
+	};
+
+	private final Log logger = LogFactory.getLog(getClass());
+
+	private final RestClient restClient;
+
+	private Converter<String, RequestEntity<?>> requestEntityConverter;
+
+	private Converter<OAuth2TokenIntrospectionClaimAccessor, ? extends OAuth2AuthenticatedPrincipal> authenticationConverter = this::defaultAuthenticationConverter;
+
+	/**
+	 * Creates a {@code OpaqueTokenAuthenticationProvider} with the provided parameters
+	 * The given {@link RestClient} should perform its own client authentication against
+	 * the introspection endpoint.
+	 * @param introspectionUri The introspection endpoint uri
+	 * @param restClient The client for performing the introspection request
+	 */
+	public RestClientSpringOpaqueTokenIntrospector(String introspectionUri, RestClient restClient) {
+		Assert.notNull(introspectionUri, "introspectionUri cannot be null");
+		Assert.notNull(restClient, "restClient cannot be null");
+		this.requestEntityConverter = this.defaultRequestEntityConverter(URI.create(introspectionUri));
+		this.restClient = restClient;
+	}
+
+	private Converter<String, RequestEntity<?>> defaultRequestEntityConverter(URI introspectionUri) {
+		return (token) -> {
+			HttpHeaders headers = requestHeaders();
+			MultiValueMap<String, String> body = requestBody(token);
+			return new RequestEntity<>(body, headers, HttpMethod.POST, introspectionUri);
+		};
+	}
+
+	private HttpHeaders requestHeaders() {
+		HttpHeaders headers = new HttpHeaders();
+		headers.setAccept(Collections.singletonList(MediaType.APPLICATION_JSON));
+		return headers;
+	}
+
+	private MultiValueMap<String, String> requestBody(String token) {
+		MultiValueMap<String, String> body = new LinkedMultiValueMap<>();
+		body.add("token", token);
+		return body;
+	}
+
+	@Override
+	public OAuth2AuthenticatedPrincipal introspect(String token) {
+		RequestEntity<?> requestEntity = this.requestEntityConverter.convert(token);
+		if (requestEntity == null) {
+			throw new OAuth2IntrospectionException("requestEntityConverter returned a null entity");
+		}
+		ResponseEntity<Map<String, Object>> responseEntity = makeRequest(requestEntity);
+		Map<String, Object> claims = adaptToNimbusResponse(responseEntity);
+		OAuth2TokenIntrospectionClaimAccessor accessor = convertClaimsSet(claims);
+		return this.authenticationConverter.convert(accessor);
+	}
+
+	/**
+	 * Sets the {@link Converter} used for converting the OAuth 2.0 access token to a
+	 * {@link RequestEntity} representation of the OAuth 2.0 token introspection request.
+	 * @param requestEntityConverter the {@link Converter} used for converting to a
+	 * {@link RequestEntity} representation of the token introspection request
+	 */
+	public void setRequestEntityConverter(Converter<String, RequestEntity<?>> requestEntityConverter) {
+		Assert.notNull(requestEntityConverter, "requestEntityConverter cannot be null");
+		this.requestEntityConverter = requestEntityConverter;
+	}
+
+	private ResponseEntity<Map<String, Object>> makeRequest(RequestEntity<?> requestEntity) {
+		try {
+			RestClient.RequestBodySpec spec = this.restClient.method(requestEntity.getMethod())
+				.uri(requestEntity.getUrl())
+				.headers((headers) -> headers.addAll(requestEntity.getHeaders()));
+			return spec.body(requestEntity.getBody()).retrieve().toEntity(STRING_OBJECT_MAP);
+		}
+		catch (Exception ex) {
+			throw new OAuth2IntrospectionException(ex.getMessage(), ex);
+		}
+	}
+
+	private Map<String, Object> adaptToNimbusResponse(ResponseEntity<Map<String, Object>> responseEntity) {
+		if (responseEntity.getStatusCode() != HttpStatus.OK) {
+			throw new OAuth2IntrospectionException(
+					"Introspection endpoint responded with " + responseEntity.getStatusCode());
+		}
+		Map<String, Object> claims = responseEntity.getBody();
+		// relying solely on the authorization server to validate this token (not checking
+		// 'exp', for example)
+		if (claims == null) {
+			return Collections.emptyMap();
+		}
+
+		boolean active = (boolean) claims.compute(OAuth2TokenIntrospectionClaimNames.ACTIVE, (k, v) -> {
+			if (v instanceof String) {
+				return Boolean.parseBoolean((String) v);
+			}
+			if (v instanceof Boolean) {
+				return v;
+			}
+			return false;
+		});
+		if (!active) {
+			this.logger.trace("Did not validate token since it is inactive");
+			throw new BadOpaqueTokenException("Provided token isn't active");
+		}
+		return claims;
+	}
+
+	private ArrayListFromStringClaimAccessor convertClaimsSet(Map<String, Object> claims) {
+		Map<String, Object> converted = new LinkedHashMap<>(claims);
+		converted.computeIfPresent(OAuth2TokenIntrospectionClaimNames.AUD, (k, v) -> {
+			if (v instanceof String) {
+				return Collections.singletonList(v);
+			}
+			return v;
+		});
+		converted.computeIfPresent(OAuth2TokenIntrospectionClaimNames.CLIENT_ID, (k, v) -> v.toString());
+		converted.computeIfPresent(OAuth2TokenIntrospectionClaimNames.EXP,
+				(k, v) -> Instant.ofEpochSecond(((Number) v).longValue()));
+		converted.computeIfPresent(OAuth2TokenIntrospectionClaimNames.IAT,
+				(k, v) -> Instant.ofEpochSecond(((Number) v).longValue()));
+		// RFC-7662 page 7 directs users to RFC-7519 for defining the values of these
+		// issuer fields.
+		// https://datatracker.ietf.org/doc/html/rfc7662#page-7
+		//
+		// RFC-7519 page 9 defines issuer fields as being 'case-sensitive' strings
+		// containing
+		// a 'StringOrURI', which is defined on page 5 as being any string, but strings
+		// containing ':'
+		// should be treated as valid URIs.
+		// https://datatracker.ietf.org/doc/html/rfc7519#section-2
+		//
+		// It is not defined however as to whether-or-not normalized URIs should be
+		// treated as the same literal
+		// value. It only defines validation itself, so to avoid potential ambiguity or
+		// unwanted side effects that
+		// may be awkward to debug, we do not want to manipulate this value. Previous
+		// versions of Spring Security
+		// would *only* allow valid URLs, which is not what we wish to achieve here.
+		converted.computeIfPresent(OAuth2TokenIntrospectionClaimNames.ISS, (k, v) -> v.toString());
+		converted.computeIfPresent(OAuth2TokenIntrospectionClaimNames.NBF,
+				(k, v) -> Instant.ofEpochSecond(((Number) v).longValue()));
+		converted.computeIfPresent(OAuth2TokenIntrospectionClaimNames.SCOPE,
+				(k, v) -> (v instanceof String s) ? new ArrayListFromString(s.split(" ")) : v);
+		return () -> converted;
+	}
+
+	/**
+	 * <p>
+	 * Sets the {@link Converter Converter&lt;OAuth2TokenIntrospectionClaimAccessor,
+	 * OAuth2AuthenticatedPrincipal&gt;} to use. Defaults to
+	 * {@link RestClientSpringOpaqueTokenIntrospector#defaultAuthenticationConverter}.
+	 * </p>
+	 * <p>
+	 * Use if you need a custom mapping of OAuth 2.0 token claims to the authenticated
+	 * principal.
+	 * </p>
+	 * @param authenticationConverter the converter
+	 * @since 7.1
+	 */
+	public void setAuthenticationConverter(
+			Converter<OAuth2TokenIntrospectionClaimAccessor, ? extends OAuth2AuthenticatedPrincipal> authenticationConverter) {
+		Assert.notNull(authenticationConverter, "converter cannot be null");
+		this.authenticationConverter = authenticationConverter;
+	}
+
+	/**
+	 * If {@link RestClientSpringOpaqueTokenIntrospector#authenticationConverter} is not
+	 * explicitly set, this default converter will be used. transforms an
+	 * {@link OAuth2TokenIntrospectionClaimAccessor} into an
+	 * {@link OAuth2AuthenticatedPrincipal} by extracting claims, mapping scopes to
+	 * authorities, and creating a principal.
+	 * @return {@link Converter Converter&lt;OAuth2TokenIntrospectionClaimAccessor,
+	 * OAuth2AuthenticatedPrincipal&gt;}
+	 * @since 7.1
+	 */
+	private OAuth2IntrospectionAuthenticatedPrincipal defaultAuthenticationConverter(
+			OAuth2TokenIntrospectionClaimAccessor accessor) {
+		Collection<GrantedAuthority> authorities = authorities(accessor.getScopes());
+		return new OAuth2IntrospectionAuthenticatedPrincipal(accessor.getClaims(), authorities);
+	}
+
+	private Collection<GrantedAuthority> authorities(List<String> scopes) {
+		if (!(scopes instanceof ArrayListFromString)) {
+			return Collections.emptyList();
+		}
+		Collection<GrantedAuthority> authorities = new ArrayList<>();
+		for (String scope : scopes) {
+			authorities.add(new SimpleGrantedAuthority(AUTHORITY_PREFIX + scope));
+		}
+		return authorities;
+	}
+
+	/**
+	 * Creates a {@code RestClientSpringOpaqueTokenIntrospector.Builder} with the given
+	 * introspection endpoint uri
+	 * @param introspectionUri The introspection endpoint uri
+	 * @return the {@link RestClientSpringOpaqueTokenIntrospector.Builder}
+	 * @since 7.1
+	 */
+	public static RestClientSpringOpaqueTokenIntrospector.Builder withIntrospectionUri(String introspectionUri) {
+		Assert.notNull(introspectionUri, "introspectionUri cannot be null");
+		return new RestClientSpringOpaqueTokenIntrospector.Builder(introspectionUri);
+	}
+
+	// gh-7563
+	private static final class ArrayListFromString extends ArrayList<String> {
+
+		@Serial
+		private static final long serialVersionUID = -1804103555781637109L;
+
+		ArrayListFromString(String... elements) {
+			super(Arrays.asList(elements));
+		}
+
+	}
+
+	// gh-15165
+	private interface ArrayListFromStringClaimAccessor extends OAuth2TokenIntrospectionClaimAccessor {
+
+		@Override
+		default List<String> getScopes() {
+			Object value = getClaims().get(OAuth2TokenIntrospectionClaimNames.SCOPE);
+			if (value instanceof ArrayListFromString list) {
+				return list;
+			}
+			return OAuth2TokenIntrospectionClaimAccessor.super.getScopes();
+		}
+
+	}
+
+	/**
+	 * Used to build {@link RestClientSpringOpaqueTokenIntrospector}.
+	 *
+	 * @author Andrey Litvitski
+	 * @since 7.1
+	 */
+	public static final class Builder {
+
+		private final String introspectionUri;
+
+		private String clientId;
+
+		private String clientSecret;
+
+		private Builder(String introspectionUri) {
+			this.introspectionUri = introspectionUri;
+		}
+
+		/**
+		 * The builder will {@link URLEncoder encode} the client id that you provide, so
+		 * please give the unencoded value.
+		 * @param clientId The unencoded client id
+		 * @return the {@link RestClientSpringOpaqueTokenIntrospector.Builder}
+		 * @since 7.1
+		 */
+		public RestClientSpringOpaqueTokenIntrospector.Builder clientId(String clientId) {
+			Assert.notNull(clientId, "clientId cannot be null");
+			this.clientId = URLEncoder.encode(clientId, StandardCharsets.UTF_8);
+			return this;
+		}
+
+		/**
+		 * The builder will {@link URLEncoder encode} the client secret that you provide,
+		 * so please give the unencoded value.
+		 * @param clientSecret The unencoded client secret
+		 * @return the {@link RestClientSpringOpaqueTokenIntrospector.Builder}
+		 * @since 7.1
+		 */
+		public RestClientSpringOpaqueTokenIntrospector.Builder clientSecret(String clientSecret) {
+			Assert.notNull(clientSecret, "clientSecret cannot be null");
+			this.clientSecret = URLEncoder.encode(clientSecret, StandardCharsets.UTF_8);
+			return this;
+		}
+
+		/**
+		 * Creates a {@code RestClientSpringOpaqueTokenIntrospector}
+		 * @return the {@link RestClientSpringOpaqueTokenIntrospector}
+		 * @since 7.1
+		 */
+		public RestClientSpringOpaqueTokenIntrospector build() {
+			RestClient restClient = RestClient.builder()
+				.defaultHeaders((headers) -> headers.setBasicAuth(this.clientId, this.clientSecret))
+				.build();
+			return new RestClientSpringOpaqueTokenIntrospector(this.introspectionUri, restClient);
+		}
+
+	}
+
+}
diff --git a/oauth2/oauth2-resource-server/src/test/java/org/springframework/security/oauth2/server/resource/introspection/RestClientSpringOpaqueTokenIntrospectorTests.java b/oauth2/oauth2-resource-server/src/test/java/org/springframework/security/oauth2/server/resource/introspection/RestClientSpringOpaqueTokenIntrospectorTests.java
new file mode 100644
index 0000000000..8b3fefdaeb
--- /dev/null
+++ b/oauth2/oauth2-resource-server/src/test/java/org/springframework/security/oauth2/server/resource/introspection/RestClientSpringOpaqueTokenIntrospectorTests.java
@@ -0,0 +1,400 @@
+/*
+ * Copyright 2004-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.security.oauth2.server.resource.introspection;
+
+import java.io.IOException;
+import java.time.Instant;
+import java.util.Arrays;
+import java.util.Base64;
+import java.util.Collection;
+import java.util.Map;
+import java.util.Optional;
+
+import com.nimbusds.jose.util.JSONObjectUtils;
+import okhttp3.mockwebserver.Dispatcher;
+import okhttp3.mockwebserver.MockResponse;
+import okhttp3.mockwebserver.MockWebServer;
+import okhttp3.mockwebserver.RecordedRequest;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.MediaType;
+import org.springframework.http.ResponseEntity;
+import org.springframework.security.core.authority.AuthorityUtils;
+import org.springframework.security.oauth2.core.OAuth2AuthenticatedPrincipal;
+import org.springframework.security.oauth2.core.OAuth2TokenIntrospectionClaimNames;
+import org.springframework.web.client.RestClient;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link RestClientSpringOpaqueTokenIntrospector}
+ *
+ * @author Andrey Litvitski
+ */
+public class RestClientSpringOpaqueTokenIntrospectorTests {
+
+	private static final String INTROSPECTION_URL = "https://server.example.com";
+
+	private static final String CLIENT_ID = "client";
+
+	private static final String CLIENT_SECRET = "secret";
+
+	// @formatter:off
+	private static final String ACTIVE_RESPONSE = "{\n"
+			+ "      \"active\": true,\n"
+			+ "      \"client_id\": \"l238j323ds-23ij4\",\n"
+			+ "      \"username\": \"jdoe\",\n"
+			+ "      \"scope\": \"read write dolphin\",\n"
+			+ "      \"sub\": \"Z5O3upPC88QrAjx00dis\",\n"
+			+ "      \"aud\": \"https://protected.example.net/resource\",\n"
+			+ "      \"iss\": \"https://server.example.com/\",\n"
+			+ "      \"exp\": 1419356238,\n"
+			+ "      \"iat\": 1419350238,\n"
+			+ "      \"extension_field\": \"twenty-seven\"\n"
+			+ "     }";
+	// @formatter:on
+
+	// @formatter:off
+	private static final String INACTIVE_RESPONSE = "{\n"
+			+ "      \"active\": false\n"
+			+ "     }";
+	// @formatter:on
+
+	// @formatter:off
+	private static final String INVALID_RESPONSE = "{\n"
+			+ "      \"client_id\": \"l238j323ds-23ij4\",\n"
+			+ "      \"username\": \"jdoe\",\n"
+			+ "      \"scope\": \"read write dolphin\",\n"
+			+ "      \"sub\": \"Z5O3upPC88QrAjx00dis\",\n"
+			+ "      \"aud\": \"https://protected.example.net/resource\",\n"
+			+ "      \"iss\": \"https://server.example.com/\",\n"
+			+ "      \"exp\": 1419356238,\n"
+			+ "      \"iat\": 1419350238,\n"
+			+ "      \"extension_field\": \"twenty-seven\"\n"
+			+ "     }";
+	// @formatter:on
+
+	// @formatter:off
+	private static final String MALFORMED_SCOPE_RESPONSE = "{\n"
+			+ "      \"active\": true,\n"
+			+ "      \"client_id\": \"l238j323ds-23ij4\",\n"
+			+ "      \"username\": \"jdoe\",\n"
+			+ "      \"scope\": [ \"read\", \"write\", \"dolphin\" ],\n"
+			+ "      \"sub\": \"Z5O3upPC88QrAjx00dis\",\n"
+			+ "      \"aud\": \"https://protected.example.net/resource\",\n"
+			+ "      \"iss\": \"https://server.example.com/\",\n"
+			+ "      \"exp\": 1419356238,\n"
+			+ "      \"iat\": 1419350238,\n"
+			+ "      \"extension_field\": \"twenty-seven\"\n"
+			+ "     }";
+	// @formatter:on
+
+	@Test
+	public void introspectWhenActiveTokenThenOk() throws Exception {
+		try (MockWebServer server = new MockWebServer()) {
+			server.setDispatcher(requiresAuth(CLIENT_ID, CLIENT_SECRET, ACTIVE_RESPONSE));
+			String introspectUri = server.url("/introspect").toString();
+			OpaqueTokenIntrospector introspectionClient = RestClientSpringOpaqueTokenIntrospector
+				.withIntrospectionUri(introspectUri)
+				.clientId(CLIENT_ID)
+				.clientSecret(CLIENT_SECRET)
+				.build();
+			OAuth2AuthenticatedPrincipal authority = introspectionClient.introspect("token");
+			// @formatter:off
+			assertThat(authority.getAttributes())
+					.isNotNull()
+					.containsEntry(OAuth2TokenIntrospectionClaimNames.ACTIVE, true)
+					.containsEntry(OAuth2TokenIntrospectionClaimNames.AUD,
+							Arrays.asList("https://protected.example.net/resource"))
+					.containsEntry(OAuth2TokenIntrospectionClaimNames.CLIENT_ID, "l238j323ds-23ij4")
+					.containsEntry(OAuth2TokenIntrospectionClaimNames.EXP, Instant.ofEpochSecond(1419356238))
+					.containsEntry(OAuth2TokenIntrospectionClaimNames.ISS, "https://server.example.com/")
+					.containsEntry(OAuth2TokenIntrospectionClaimNames.SCOPE, Arrays.asList("read", "write", "dolphin"))
+					.containsEntry(OAuth2TokenIntrospectionClaimNames.SUB, "Z5O3upPC88QrAjx00dis")
+					.containsEntry(OAuth2TokenIntrospectionClaimNames.USERNAME, "jdoe")
+					.containsEntry("extension_field", "twenty-seven");
+			// @formatter:on
+		}
+	}
+
+	@Test
+	public void introspectWhenBadClientCredentialsThenError() throws IOException {
+		try (MockWebServer server = new MockWebServer()) {
+			server.setDispatcher(requiresAuth(CLIENT_ID, CLIENT_SECRET, ACTIVE_RESPONSE));
+			String introspectUri = server.url("/introspect").toString();
+			OpaqueTokenIntrospector introspectionClient = RestClientSpringOpaqueTokenIntrospector
+				.withIntrospectionUri(introspectUri)
+				.clientId(CLIENT_ID)
+				.clientSecret("wrong")
+				.build();
+			assertThatExceptionOfType(OAuth2IntrospectionException.class)
+				.isThrownBy(() -> introspectionClient.introspect("token"));
+		}
+	}
+
+	@Test
+	public void introspectWhenInactiveTokenThenInvalidToken() throws Exception {
+		try (MockWebServer server = new MockWebServer()) {
+			server.setDispatcher(requiresAuth(CLIENT_ID, CLIENT_SECRET, INACTIVE_RESPONSE));
+			String introspectUri = server.url("/introspect").toString();
+			OpaqueTokenIntrospector introspectionClient = RestClientSpringOpaqueTokenIntrospector
+				.withIntrospectionUri(introspectUri)
+				.clientId(CLIENT_ID)
+				.clientSecret(CLIENT_SECRET)
+				.build();
+
+			assertThatExceptionOfType(OAuth2IntrospectionException.class)
+				.isThrownBy(() -> introspectionClient.introspect("token"))
+				.withMessage("Provided token isn't active");
+		}
+	}
+
+	@Test
+	public void introspectWhenActiveTokenThenParsesValuesInResponse() throws Exception {
+		try (MockWebServer server = new MockWebServer()) {
+			String response = """
+					{
+					  "active": true,
+					  "aud": ["aud"],
+					  "nbf": 29348723984
+					}
+					""";
+			server.setDispatcher(requiresAuth(CLIENT_ID, CLIENT_SECRET, response));
+			String introspectUri = server.url("/introspect").toString();
+			OpaqueTokenIntrospector introspectionClient = RestClientSpringOpaqueTokenIntrospector
+				.withIntrospectionUri(introspectUri)
+				.clientId(CLIENT_ID)
+				.clientSecret(CLIENT_SECRET)
+				.build();
+			OAuth2AuthenticatedPrincipal authority = introspectionClient.introspect("token");
+			assertThat(authority.getAttributes()).isNotNull()
+				.containsEntry(OAuth2TokenIntrospectionClaimNames.ACTIVE, true)
+				.containsEntry(OAuth2TokenIntrospectionClaimNames.AUD, Arrays.asList("aud"))
+				.containsEntry(OAuth2TokenIntrospectionClaimNames.NBF, Instant.ofEpochSecond(29348723984L))
+				.doesNotContainKey(OAuth2TokenIntrospectionClaimNames.CLIENT_ID)
+				.doesNotContainKey(OAuth2TokenIntrospectionClaimNames.SCOPE);
+		}
+	}
+
+	@Test
+	public void introspectWhenIntrospectionEndpointThrowsExceptionThenInvalidToken() throws Exception {
+		try (MockWebServer server = new MockWebServer()) {
+			server.setDispatcher(requiresAuth(CLIENT_ID, CLIENT_SECRET, ACTIVE_RESPONSE));
+			server.start();
+			String introspectUri = server.url("/introspect").toString();
+			server.shutdown();
+			OpaqueTokenIntrospector introspectionClient = RestClientSpringOpaqueTokenIntrospector
+				.withIntrospectionUri(introspectUri)
+				.clientId(CLIENT_ID)
+				.clientSecret(CLIENT_SECRET)
+				.build();
+			assertThatExceptionOfType(OAuth2IntrospectionException.class)
+				.isThrownBy(() -> introspectionClient.introspect("token"));
+		}
+	}
+
+	@Test
+	public void introspectWhenIntrospectionEndpointReturnsMalformedResponseThenInvalidToken() throws Exception {
+		try (MockWebServer server = new MockWebServer()) {
+			server.setDispatcher(requiresAuth(CLIENT_ID, CLIENT_SECRET, "{}"));
+			String introspectUri = server.url("/introspect").toString();
+			OpaqueTokenIntrospector introspectionClient = RestClientSpringOpaqueTokenIntrospector
+				.withIntrospectionUri(introspectUri)
+				.clientId(CLIENT_ID)
+				.clientSecret(CLIENT_SECRET)
+				.build();
+			assertThatExceptionOfType(OAuth2IntrospectionException.class)
+				.isThrownBy(() -> introspectionClient.introspect("token"));
+		}
+	}
+
+	@Test
+	public void introspectWhenIntrospectionTokenReturnsInvalidResponseThenInvalidToken() throws Exception {
+		try (MockWebServer server = new MockWebServer()) {
+			server.setDispatcher(requiresAuth(CLIENT_ID, CLIENT_SECRET, INVALID_RESPONSE));
+			String introspectUri = server.url("/introspect").toString();
+			OpaqueTokenIntrospector introspectionClient = RestClientSpringOpaqueTokenIntrospector
+				.withIntrospectionUri(introspectUri)
+				.clientId(CLIENT_ID)
+				.clientSecret(CLIENT_SECRET)
+				.build();
+			assertThatExceptionOfType(OAuth2IntrospectionException.class)
+				.isThrownBy(() -> introspectionClient.introspect("token"));
+		}
+	}
+
+	// gh-7563
+	@Test
+	public void introspectWhenIntrospectionTokenReturnsMalformedScopeThenEmptyAuthorities() throws Exception {
+		try (MockWebServer server = new MockWebServer()) {
+			server.setDispatcher(requiresAuth(CLIENT_ID, CLIENT_SECRET, MALFORMED_SCOPE_RESPONSE));
+			String introspectUri = server.url("/introspect").toString();
+			OpaqueTokenIntrospector introspectionClient = RestClientSpringOpaqueTokenIntrospector
+				.withIntrospectionUri(introspectUri)
+				.clientId(CLIENT_ID)
+				.clientSecret(CLIENT_SECRET)
+				.build();
+			OAuth2AuthenticatedPrincipal principal = introspectionClient.introspect("token");
+			assertThat(principal.getAuthorities()).isEmpty();
+			Collection<String> scope = principal.getAttribute("scope");
+			assertThat(scope).containsExactly("read", "write", "dolphin");
+		}
+	}
+
+	// gh-15165
+	@Test
+	public void introspectWhenActiveThenMapsAuthorities() throws Exception {
+		try (MockWebServer server = new MockWebServer()) {
+			server.setDispatcher(requiresAuth(CLIENT_ID, CLIENT_SECRET, ACTIVE_RESPONSE));
+			String introspectUri = server.url("/introspect").toString();
+			OpaqueTokenIntrospector introspectionClient = RestClientSpringOpaqueTokenIntrospector
+				.withIntrospectionUri(introspectUri)
+				.clientId(CLIENT_ID)
+				.clientSecret(CLIENT_SECRET)
+				.build();
+			OAuth2AuthenticatedPrincipal principal = introspectionClient.introspect("token");
+			assertThat(principal.getAuthorities()).isNotEmpty();
+			Collection<String> scope = principal.getAttribute("scope");
+			assertThat(scope).containsExactly("read", "write", "dolphin");
+			Collection<String> authorities = AuthorityUtils.authorityListToSet(principal.getAuthorities());
+			assertThat(authorities).containsExactly("SCOPE_read", "SCOPE_write", "SCOPE_dolphin");
+		}
+	}
+
+	@Test
+	public void setRequestEntityConverterWhenConverterIsNullThenExceptionIsThrown() {
+		RestClient restClient = mock(RestClient.class);
+		RestClientSpringOpaqueTokenIntrospector introspectionClient = new RestClientSpringOpaqueTokenIntrospector(
+				INTROSPECTION_URL, restClient);
+		assertThatExceptionOfType(IllegalArgumentException.class)
+			.isThrownBy(() -> introspectionClient.setRequestEntityConverter(null));
+	}
+
+	@Test
+	public void setAuthenticationConverterWhenConverterIsNullThenExceptionIsThrown() {
+		RestClient restClient = mock(RestClient.class);
+		RestClientSpringOpaqueTokenIntrospector introspectionClient = new RestClientSpringOpaqueTokenIntrospector(
+				INTROSPECTION_URL, restClient);
+		assertThatExceptionOfType(IllegalArgumentException.class)
+			.isThrownBy(() -> introspectionClient.setAuthenticationConverter(null));
+	}
+
+	@Test
+	public void introspectWithoutEncodeClientCredentialsThenExceptionIsThrown() throws Exception {
+		try (MockWebServer server = new MockWebServer()) {
+			String response = """
+					{
+						"active": true,
+						"username": "client%&1"
+					}
+					""";
+			server.setDispatcher(requiresAuth("client%25%261", "secret%40%242", response));
+			String introspectUri = server.url("/introspect").toString();
+			RestClient restClient = RestClient.builder()
+				.defaultHeaders((h) -> h.setBasicAuth("client%&1", "secret@$2"))
+				.build();
+			OpaqueTokenIntrospector introspectionClient = new RestClientSpringOpaqueTokenIntrospector(introspectUri,
+					restClient);
+			assertThatExceptionOfType(OAuth2IntrospectionException.class)
+				.isThrownBy(() -> introspectionClient.introspect("token"));
+		}
+	}
+
+	@Test
+	public void introspectWithEncodeClientCredentialsThenOk() throws Exception {
+		try (MockWebServer server = new MockWebServer()) {
+			String response = """
+					{
+						"active": true,
+						"username": "client&1"
+					}
+					""";
+			server.setDispatcher(requiresAuth("client%261", "secret%40%242", response));
+			String introspectUri = server.url("/introspect").toString();
+			OpaqueTokenIntrospector introspectionClient = SpringOpaqueTokenIntrospector
+				.withIntrospectionUri(introspectUri)
+				.clientId("client&1")
+				.clientSecret("secret@$2")
+				.build();
+			OAuth2AuthenticatedPrincipal authority = introspectionClient.introspect("token");
+			// @formatter:off
+			assertThat(authority.getAttributes())
+					.isNotNull()
+					.containsEntry(OAuth2TokenIntrospectionClaimNames.ACTIVE, true)
+					.containsEntry(OAuth2TokenIntrospectionClaimNames.USERNAME, "client&1");
+			// @formatter:on
+		}
+	}
+
+	private static ResponseEntity<Map<String, Object>> response(String content) {
+		HttpHeaders headers = new HttpHeaders();
+		headers.setContentType(MediaType.APPLICATION_JSON);
+		try {
+			return new ResponseEntity<>(JSONObjectUtils.parse(content), headers, HttpStatus.OK);
+		}
+		catch (Exception ex) {
+			throw new IllegalArgumentException(ex);
+		}
+	}
+
+	private static ResponseEntity<Map<String, Object>> response(Map<String, Object> content) {
+		HttpHeaders headers = new HttpHeaders();
+		headers.setContentType(MediaType.APPLICATION_JSON);
+		try {
+			return new ResponseEntity<>(content, headers, HttpStatus.OK);
+		}
+		catch (Exception ex) {
+			throw new IllegalArgumentException(ex);
+		}
+	}
+
+	private static Dispatcher requiresAuth(String username, String password, String response) {
+		return new Dispatcher() {
+			@Override
+			public MockResponse dispatch(RecordedRequest request) {
+				String authorization = request.getHeader(HttpHeaders.AUTHORIZATION);
+				// @formatter:off
+				return Optional.ofNullable(authorization)
+						.filter((a) -> isAuthorized(authorization, username, password))
+						.map((a) -> ok(response))
+						.orElse(unauthorized());
+				// @formatter:on
+			}
+		};
+	}
+
+	private static boolean isAuthorized(String authorization, String username, String password) {
+		String[] values = new String(Base64.getDecoder().decode(authorization.substring(6))).split(":");
+		return username.equals(values[0]) && password.equals(values[1]);
+	}
+
+	private static MockResponse ok(String response) {
+		// @formatter:off
+		return new MockResponse().setBody(response)
+				.setHeader(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE);
+		// @formatter:on
+	}
+
+	private static MockResponse unauthorized() {
+		return new MockResponse().setResponseCode(401);
+	}
+
+}