diff --git a/extensions-core/s3-extensions/src/test/java/org/apache/druid/storage/s3/output/RetryableS3OutputStreamTest.java b/extensions-core/s3-extensions/src/test/java/org/apache/druid/storage/s3/output/RetryableS3OutputStreamTest.java
index 8e7a81eb48dd..e02701e90397 100644
--- a/extensions-core/s3-extensions/src/test/java/org/apache/druid/storage/s3/output/RetryableS3OutputStreamTest.java
+++ b/extensions-core/s3-extensions/src/test/java/org/apache/druid/storage/s3/output/RetryableS3OutputStreamTest.java
@@ -113,12 +113,8 @@ public void testWriteAndHappy() throws IOException
   {
     chunkSize = 10;
     ByteBuffer bb = ByteBuffer.allocate(Integer.BYTES);
-    try (RetryableS3OutputStream out = new RetryableS3OutputStream(
-        config,
-        s3,
-        path,
-        s3UploadManager
-    )) {
+    try (RetryableS3OutputStream out =
+             new RetryableS3OutputStream(config, s3, path, s3UploadManager)) {
       for (int i = 0; i < 25; i++) {
         bb.clear();
         bb.putInt(i);
@@ -135,12 +131,8 @@ public void testWriteSizeLargerThanConfiguredMaxChunkSizeShouldSucceed() throws
   {
     chunkSize = 10;
     ByteBuffer bb = ByteBuffer.allocate(Integer.BYTES * 3);
-    try (RetryableS3OutputStream out = new RetryableS3OutputStream(
-        config,
-        s3,
-        path,
-        s3UploadManager
-    )) {
+    try (RetryableS3OutputStream out =
+             new RetryableS3OutputStream(config, s3, path, s3UploadManager)) {
       bb.clear();
       bb.putInt(1);
       bb.putInt(2);
@@ -156,12 +148,8 @@ public void testWriteSizeLargerThanConfiguredMaxChunkSizeShouldSucceed() throws
   public void testWriteSmallBufferShouldSucceed() throws IOException
   {
     chunkSize = 128;
-    try (RetryableS3OutputStream out = new RetryableS3OutputStream(
-        config,
-        s3,
-        path,
-        s3UploadManager
-    )) {
+    try (RetryableS3OutputStream out =
+             new RetryableS3OutputStream(config, s3, path, s3UploadManager)) {
       for (int i = 0; i < 600; i++) {
         out.write(i);
       }
@@ -171,6 +159,22 @@ public void testWriteSmallBufferShouldSucceed() throws IOException
     s3.assertCompleted(chunkSize, 600);
   }
 
+  @Test
+  public void testWriteSmallBufferExactChunkSizeShouldSucceed() throws IOException
+  {
+    chunkSize = 128;
+    final int fileSize = 128 * 5;
+    try (RetryableS3OutputStream out =
+             new RetryableS3OutputStream(config, s3, path, s3UploadManager)) {
+      for (int i = 0; i < fileSize; i++) {
+        out.write(i);
+      }
+    }
+    // each chunk 128 bytes, so there should be 5 chunks.
+    Assert.assertEquals(5, s3.partRequests.size());
+    s3.assertCompleted(chunkSize, fileSize);
+  }
+
   @Test
   public void testSuccessToUploadAfterRetry() throws IOException
   {
@@ -178,12 +182,8 @@ public void testSuccessToUploadAfterRetry() throws IOException
 
     chunkSize = 10;
     ByteBuffer bb = ByteBuffer.allocate(Integer.BYTES);
-    try (RetryableS3OutputStream out = new RetryableS3OutputStream(
-        config,
-        s3,
-        path,
-        s3UploadManager
-    )) {
+    try (RetryableS3OutputStream out =
+             new RetryableS3OutputStream(config, s3, path, s3UploadManager)) {
       for (int i = 0; i < 25; i++) {
         bb.clear();
         bb.putInt(i);
@@ -201,12 +201,8 @@ public void testFailToUploadAfterRetries() throws IOException
     final TestAmazonS3 s3 = new TestAmazonS3(3);
 
     ByteBuffer bb = ByteBuffer.allocate(Integer.BYTES);
-    try (RetryableS3OutputStream out = new RetryableS3OutputStream(
-        config,
-        s3,
-        path,
-        s3UploadManager
-    )) {
+    try (RetryableS3OutputStream out =
+             new RetryableS3OutputStream(config, s3, path, s3UploadManager)) {
       for (int i = 0; i < 2; i++) {
         bb.clear();
         bb.putInt(i);
@@ -284,13 +280,16 @@ private void assertCompleted(long chunkSize, long expectedFileSize)
       Set<Integer> partNumbersFromRequest = partRequests.stream().map(UploadPartRequest::getPartNumber).collect(Collectors.toSet());
       Assert.assertEquals(partRequests.size(), partNumbersFromRequest.size());
 
-      for (int i = 0; i < partRequests.size(); i++) {
-        if (i < partRequests.size() - 1) {
-          Assert.assertEquals(chunkSize, partRequests.get(i).getPartSize());
-        } else {
-          Assert.assertTrue(chunkSize >= partRequests.get(i).getPartSize());
+      // Verify sizes of uploaded chunks
+      int numSmallerChunks = 0;
+      for (UploadPartRequest part : partRequests) {
+        Assert.assertTrue(part.getPartSize() <= chunkSize);
+        if (part.getPartSize() < chunkSize) {
+          ++numSmallerChunks;
         }
       }
+      Assert.assertTrue(numSmallerChunks <= 1);
+
       final List<PartETag> eTags = completeRequest.getPartETags();
       Assert.assertEquals(partRequests.size(), eTags.size());
       Assert.assertEquals(