diff --git a/docs/plugins/repository-s3.asciidoc b/docs/plugins/repository-s3.asciidoc
index a98c9a525d8..46789cf3f6e 100644
--- a/docs/plugins/repository-s3.asciidoc
+++ b/docs/plugins/repository-s3.asciidoc
@@ -217,7 +217,8 @@ The following settings are supported:
     to split the chunk into several parts, each of `buffer_size` length, and
     to upload each part in its own request. Note that setting a buffer
     size lower than `5mb` is not allowed since it will prevents the use of the
-    Multipart API and may result in upload errors. Defaults to `100mb`.
+    Multipart API and may result in upload errors. Defaults to the minimum
+    between `100mb` and `5%` of the heap size.
 
 `max_retries`::
 
diff --git a/plugins/repository-s3/src/main/java/org/elasticsearch/repositories/s3/S3Repository.java b/plugins/repository-s3/src/main/java/org/elasticsearch/repositories/s3/S3Repository.java
index 34e4d78f8cf..7310b527158 100644
--- a/plugins/repository-s3/src/main/java/org/elasticsearch/repositories/s3/S3Repository.java
+++ b/plugins/repository-s3/src/main/java/org/elasticsearch/repositories/s3/S3Repository.java
@@ -33,6 +33,7 @@ import org.elasticsearch.common.settings.Setting.Property;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.ByteSizeUnit;
 import org.elasticsearch.common.unit.ByteSizeValue;
+import org.elasticsearch.monitor.jvm.JvmInfo;
 import org.elasticsearch.repositories.RepositoryException;
 import org.elasticsearch.repositories.blobstore.BlobStoreRepository;
 
@@ -101,14 +102,27 @@ public class S3Repository extends BlobStoreRepository {
          */
         Setting<Boolean> SERVER_SIDE_ENCRYPTION_SETTING =
             Setting.boolSetting("repositories.s3.server_side_encryption", false, Property.NodeScope);
+
+        /**
+         * Default is to use 100MB (S3 defaults) for heaps above 2GB and 5% of
+         * the available memory for smaller heaps.
+         */
+        ByteSizeValue DEFAULT_BUFFER_SIZE = new ByteSizeValue(
+                Math.max(
+                        ByteSizeUnit.MB.toBytes(5), // minimum value
+                        Math.min(
+                                ByteSizeUnit.MB.toBytes(100),
+                                JvmInfo.jvmInfo().getMem().getHeapMax().getBytes() / 20)),
+                ByteSizeUnit.BYTES);
+
         /**
          * repositories.s3.buffer_size: Minimum threshold below which the chunk is uploaded using a single request. Beyond this threshold,
          * the S3 repository will use the AWS Multipart Upload API to split the chunk into several parts, each of buffer_size length, and
          * to upload each part in its own request. Note that setting a buffer size lower than 5mb is not allowed since it will prevents the
-         * use of the Multipart API and may result in upload errors. Defaults to 100m.
+         * use of the Multipart API and may result in upload errors. Defaults to the minimum between 100MB and 5% of the heap size.
          */
         Setting<ByteSizeValue> BUFFER_SIZE_SETTING =
-            Setting.byteSizeSetting("repositories.s3.buffer_size", new ByteSizeValue(100, ByteSizeUnit.MB),
+            Setting.byteSizeSetting("repositories.s3.buffer_size", DEFAULT_BUFFER_SIZE,
                 new ByteSizeValue(5, ByteSizeUnit.MB), new ByteSizeValue(5, ByteSizeUnit.TB), Property.NodeScope);
         /**
          * repositories.s3.max_retries: Number of retries in case of S3 errors. Defaults to 3.
@@ -195,12 +209,13 @@ public class S3Repository extends BlobStoreRepository {
          * @see  Repositories#SERVER_SIDE_ENCRYPTION_SETTING
          */
         Setting<Boolean> SERVER_SIDE_ENCRYPTION_SETTING = Setting.boolSetting("server_side_encryption", false);
+
         /**
          * buffer_size
          * @see  Repositories#BUFFER_SIZE_SETTING
          */
         Setting<ByteSizeValue> BUFFER_SIZE_SETTING =
-            Setting.byteSizeSetting("buffer_size", new ByteSizeValue(100, ByteSizeUnit.MB),
+            Setting.byteSizeSetting("buffer_size", Repositories.DEFAULT_BUFFER_SIZE,
                 new ByteSizeValue(5, ByteSizeUnit.MB), new ByteSizeValue(5, ByteSizeUnit.TB));
         /**
          * max_retries
diff --git a/plugins/repository-s3/src/test/java/org/elasticsearch/repositories/s3/S3RepositoryTests.java b/plugins/repository-s3/src/test/java/org/elasticsearch/repositories/s3/S3RepositoryTests.java
index 915183888b6..14595d13448 100644
--- a/plugins/repository-s3/src/test/java/org/elasticsearch/repositories/s3/S3RepositoryTests.java
+++ b/plugins/repository-s3/src/test/java/org/elasticsearch/repositories/s3/S3RepositoryTests.java
@@ -32,6 +32,7 @@ import org.elasticsearch.common.unit.ByteSizeUnit;
 import org.elasticsearch.common.unit.ByteSizeValue;
 import org.elasticsearch.repositories.RepositoryException;
 import org.elasticsearch.test.ESTestCase;
+import org.hamcrest.Matchers;
 
 import static org.elasticsearch.repositories.s3.S3Repository.Repositories;
 import static org.elasticsearch.repositories.s3.S3Repository.Repository;
@@ -117,4 +118,12 @@ public class S3RepositoryTests extends ESTestCase {
         assertEquals("foo/bar/", s3repo.basePath().buildAsString()); // make sure leading `/` is removed and trailing is added
     }
 
+    public void testDefaultBufferSize() {
+        ByteSizeValue defaultBufferSize = S3Repository.Repository.BUFFER_SIZE_SETTING.get(Settings.EMPTY);
+        assertThat(defaultBufferSize, Matchers.lessThanOrEqualTo(new ByteSizeValue(100, ByteSizeUnit.MB)));
+        assertThat(defaultBufferSize, Matchers.greaterThanOrEqualTo(new ByteSizeValue(5, ByteSizeUnit.MB)));
+
+        ByteSizeValue defaultNodeBufferSize = S3Repository.Repositories.BUFFER_SIZE_SETTING.get(Settings.EMPTY);
+        assertEquals(defaultBufferSize, defaultNodeBufferSize);
+    }
 }