diff --git a/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/registration/InMemoryReactiveClientRegistrationRepository.java b/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/registration/InMemoryReactiveClientRegistrationRepository.java
index 4d2db9d0a6..98ba597c41 100644
--- a/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/registration/InMemoryReactiveClientRegistrationRepository.java
+++ b/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/registration/InMemoryReactiveClientRegistrationRepository.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -15,15 +15,22 @@
  */
 package org.springframework.security.oauth2.client.registration;
 
+import java.util.Arrays;
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 import reactor.core.publisher.Mono;
 
+import org.springframework.util.Assert;
+
 /**
  * A Reactive {@link ClientRegistrationRepository} that stores {@link ClientRegistration}(s) in-memory.
  *
  * @author Rob Winch
+ * @author Ebert Toribio
  * @since 5.1
  * @see ClientRegistrationRepository
  * @see ClientRegistration
@@ -31,7 +38,7 @@ import reactor.core.publisher.Mono;
 public final class InMemoryReactiveClientRegistrationRepository
 		implements ReactiveClientRegistrationRepository, Iterable<ClientRegistration> {
 
-	private final InMemoryClientRegistrationRepository delegate;
+	private final Map<String, ClientRegistration> clientIdToClientRegistration;
 
 	/**
 	 * Constructs an {@code InMemoryReactiveClientRegistrationRepository} using the provided parameters.
@@ -39,7 +46,12 @@ public final class InMemoryReactiveClientRegistrationRepository
 	 * @param registrations the client registration(s)
 	 */
 	public InMemoryReactiveClientRegistrationRepository(ClientRegistration... registrations) {
-		this.delegate = new InMemoryClientRegistrationRepository(registrations);
+		this(toList(registrations));
+	}
+
+	private static List<ClientRegistration> toList(ClientRegistration... registrations) {
+		Assert.notEmpty(registrations, "registrations cannot be null or empty");
+		return Arrays.asList(registrations);
 	}
 
 	/**
@@ -48,12 +60,12 @@ public final class InMemoryReactiveClientRegistrationRepository
 	 * @param registrations the client registration(s)
 	 */
 	public InMemoryReactiveClientRegistrationRepository(List<ClientRegistration> registrations) {
-		this.delegate = new InMemoryClientRegistrationRepository(registrations);
+		this.clientIdToClientRegistration = toUnmodifiableConcurrentMap(registrations);
 	}
 
 	@Override
 	public Mono<ClientRegistration> findByRegistrationId(String registrationId) {
-		return Mono.justOrEmpty(this.delegate.findByRegistrationId(registrationId));
+		return Mono.justOrEmpty(this.clientIdToClientRegistration.get(registrationId));
 	}
 
 	/**
@@ -63,6 +75,20 @@ public final class InMemoryReactiveClientRegistrationRepository
 	 */
 	@Override
 	public Iterator<ClientRegistration> iterator() {
-		return delegate.iterator();
+		return this.clientIdToClientRegistration.values().iterator();
+	}
+
+	private static Map<String, ClientRegistration> toUnmodifiableConcurrentMap(List<ClientRegistration> registrations) {
+		Assert.notEmpty(registrations, "registrations cannot be null or empty");
+		ConcurrentHashMap<String, ClientRegistration> result = new ConcurrentHashMap<>();
+		for (ClientRegistration registration : registrations) {
+			Assert.notNull(registration, "no registration can be null");
+			if (result.containsKey(registration.getRegistrationId())) {
+				throw new IllegalStateException(String.format("Duplicate key %s",
+						registration.getRegistrationId()));
+			}
+			result.put(registration.getRegistrationId(), registration);
+		}
+		return Collections.unmodifiableMap(result);
 	}
 }