diff --git a/feature/s3/manager/upload.go b/feature/s3/manager/upload.go
index 93133aee585..a287489d7d6 100644
--- a/feature/s3/manager/upload.go
+++ b/feature/s3/manager/upload.go
@@ -341,6 +341,18 @@ type uploader struct {
 	totalSize int64 // set to -1 if the size is not known
 }
 
+// getRequestChecksumCalculation extracts the RequestChecksumCalculation setting
+// from the uploader's ClientOptions configuration.
+func (u *uploader) getRequestChecksumCalculation() aws.RequestChecksumCalculation {
+	opts := &s3.Options{}
+
+	for _, opt := range u.cfg.ClientOptions {
+		opt(opts)
+	}
+
+	return opts.RequestChecksumCalculation
+}
+
 // internal logic for deciding whether to upload a single part or use a
 // multipart upload.
 func (u *uploader) upload() (*UploadOutput, error) {
@@ -776,7 +788,11 @@ func (u *multiuploader) initChecksumAlgorithm() {
 	case u.in.ChecksumSHA256 != nil:
 		u.in.ChecksumAlgorithm = types.ChecksumAlgorithmSha256
 	default:
-		u.in.ChecksumAlgorithm = types.ChecksumAlgorithmCrc32
+		checksumCalc := u.getRequestChecksumCalculation()
+
+		if checksumCalc != aws.RequestChecksumCalculationWhenRequired {
+			u.in.ChecksumAlgorithm = types.ChecksumAlgorithmCrc32
+		}
 	}
 }
 
diff --git a/feature/s3/manager/upload_test.go b/feature/s3/manager/upload_test.go
index ecbafb755d7..0fde5d8dcaa 100644
--- a/feature/s3/manager/upload_test.go
+++ b/feature/s3/manager/upload_test.go
@@ -1160,6 +1160,95 @@ func (h *failPartHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	h.failsRemaining--
 }
 
+// TestUploadRequestChecksumCalculation tests that checksum behavior respects
+// the RequestChecksumCalculation setting for backwards compatibility.
+func TestUploadRequestChecksumCalculation(t *testing.T) {
+	testCases := []struct {
+		name                       string
+		requestChecksumCalculation aws.RequestChecksumCalculation
+		inputChecksumAlgorithm     types.ChecksumAlgorithm
+		expectedChecksumAlgorithm  types.ChecksumAlgorithm
+		description                string
+	}{
+		{
+			name:                       "WhenRequired_NoDefault",
+			requestChecksumCalculation: aws.RequestChecksumCalculationWhenRequired,
+			inputChecksumAlgorithm:     "",
+			expectedChecksumAlgorithm:  "",
+			description:                "When RequestChecksumCalculationWhenRequired is set, no default checksum should be applied (backwards compatibility)",
+		},
+		{
+			name:                       "WhenSupported_HasDefault",
+			requestChecksumCalculation: aws.RequestChecksumCalculationWhenSupported,
+			inputChecksumAlgorithm:     "",
+			expectedChecksumAlgorithm:  types.ChecksumAlgorithmCrc32,
+			description:                "When RequestChecksumCalculationWhenSupported is set, default CRC32 checksum should be applied",
+		},
+		{
+			name:                       "WhenRequired_ExplicitPreserved",
+			requestChecksumCalculation: aws.RequestChecksumCalculationWhenRequired,
+			inputChecksumAlgorithm:     types.ChecksumAlgorithmSha256,
+			expectedChecksumAlgorithm:  types.ChecksumAlgorithmSha256,
+			description:                "Explicit checksums should always be preserved regardless of RequestChecksumCalculation setting",
+		},
+		{
+			name:                       "WhenSupported_ExplicitPreserved",
+			requestChecksumCalculation: aws.RequestChecksumCalculationWhenSupported,
+			inputChecksumAlgorithm:     types.ChecksumAlgorithmSha1,
+			expectedChecksumAlgorithm:  types.ChecksumAlgorithmSha1,
+			description:                "Explicit checksums should override defaults even with WhenSupported",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			c, invocations, args := s3testing.NewUploadLoggingClient(nil)
+
+			mgr := manager.NewUploader(c,
+				manager.WithUploaderRequestOptions(func(o *s3.Options) {
+					o.RequestChecksumCalculation = tc.requestChecksumCalculation
+				}))
+
+			input := &s3.PutObjectInput{
+				Bucket: aws.String("Bucket"),
+				Key:    aws.String("Key"),
+				Body:   bytes.NewReader(buf12MB), // Large enough to trigger multipart upload
+			}
+			if tc.inputChecksumAlgorithm != "" {
+				input.ChecksumAlgorithm = tc.inputChecksumAlgorithm
+			}
+
+			resp, err := mgr.Upload(context.Background(), input)
+			if err != nil {
+				t.Errorf("Expected no error but received %v", err)
+			}
+
+			expectedOps := []string{"CreateMultipartUpload", "UploadPart", "UploadPart", "UploadPart", "CompleteMultipartUpload"}
+			if diff := cmpDiff(expectedOps, *invocations); len(diff) > 0 {
+				t.Error(diff)
+			}
+
+			if resp.UploadID != "UPLOAD-ID" {
+				t.Errorf("expect %q, got %q", "UPLOAD-ID", resp.UploadID)
+			}
+
+			cmu := (*args)[0].(*s3.CreateMultipartUploadInput)
+			if cmu.ChecksumAlgorithm != tc.expectedChecksumAlgorithm {
+				t.Errorf("%s: Expected checksum algorithm %v in CreateMultipartUpload, but got %v",
+					tc.description, tc.expectedChecksumAlgorithm, cmu.ChecksumAlgorithm)
+			}
+
+			for i := 1; i <= 3; i++ {
+				uploadPart := (*args)[i].(*s3.UploadPartInput)
+				if uploadPart.ChecksumAlgorithm != tc.expectedChecksumAlgorithm {
+					t.Errorf("%s: Expected checksum algorithm %v in UploadPart %d, but got %v",
+						tc.description, tc.expectedChecksumAlgorithm, i, uploadPart.ChecksumAlgorithm)
+				}
+			}
+		})
+	}
+}
+
 type recordedBufferProvider struct {
 	content       []byte
 	size          int