diff --git a/cmd/gateway/s3/gateway-s3.go b/cmd/gateway/s3/gateway-s3.go
index 97417f57ee2b5..94793ee65b4db 100644
--- a/cmd/gateway/s3/gateway-s3.go
+++ b/cmd/gateway/s3/gateway-s3.go
@@ -17,7 +17,10 @@
 package s3
 
 import (
+	"bytes"
 	"context"
+	"crypto/md5"
+	"encoding/base64"
 	"encoding/json"
 	"io"
 	"math/rand"
@@ -520,7 +523,25 @@ func (l *s3Objects) PutObject(ctx context.Context, bucket string, object string,
 		// we can set md5sum to be calculated always.
 		SendContentMd5: true,
 	}
-	ui, err := l.Client.PutObject(ctx, bucket, object, data, data.Size(), data.MD5Base64String(), data.SHA256HexString(), putOpts)
+
+	// Core.PutObject passes md5Base64 directly to AWS without computing it.
+	// When the caller uses streaming SigV4 (no Content-MD5 header, e.g. Java
+	// AWS SDK / OpenSearch), MD5Base64String() returns "". Object Lock buckets
+	// require Content-MD5, so buffer the data and compute MD5 when missing.
+	md5Base64 := data.MD5Base64String()
+	sha256Hex := data.SHA256HexString()
+	var reader io.Reader = data
+	if md5Base64 == "" {
+		buf, err := io.ReadAll(data)
+		if err != nil {
+			return objInfo, minio.ErrorRespToObjectError(err, bucket, object)
+		}
+		h := md5.New()
+		h.Write(buf)
+		md5Base64 = base64.StdEncoding.EncodeToString(h.Sum(nil))
+		reader = bytes.NewReader(buf)
+	}
+	ui, err := l.Client.PutObject(ctx, bucket, object, reader, data.Size(), md5Base64, sha256Hex, putOpts)
 	if err != nil {
 		return objInfo, minio.ErrorRespToObjectError(err, bucket, object)
 	}
@@ -629,12 +650,31 @@ func (l *s3Objects) NewMultipartUpload(ctx context.Context, bucket string, objec
 // PutObjectPart puts a part of object in bucket
 func (l *s3Objects) PutObjectPart(ctx context.Context, bucket string, object string, uploadID string, partID int, r *minio.PutObjReader, opts minio.ObjectOptions) (pi minio.PartInfo, e error) {
 	data := r.Reader
+
+	md5Base64 := data.MD5Base64String()
+	sha256Hex := data.SHA256HexString()
+	var reader io.Reader = data
+
+	// AWS S3 requires Content-MD5 on every UploadPart request for Object Lock
+	// buckets. When the caller uses streaming SigV4 (no Content-MD5 header),
+	// MD5Base64String() returns "". Buffer the part and compute MD5 ourselves.
+	if md5Base64 == "" {
+		buf, err := io.ReadAll(data)
+		if err != nil {
+			return pi, minio.ErrorRespToObjectError(err, bucket, object)
+		}
+		h := md5.New()
+		h.Write(buf)
+		md5Base64 = base64.StdEncoding.EncodeToString(h.Sum(nil))
+		reader = bytes.NewReader(buf)
+	}
+
 	putOpts := miniogo.PutObjectPartOptions{
 		SSE:       opts.ServerSideEncryption,
-		Md5Base64: data.MD5Base64String(),
-		Sha256Hex: data.SHA256HexString(),
+		Md5Base64: md5Base64,
+		Sha256Hex: sha256Hex,
 	}
-	info, err := l.Client.PutObjectPart(ctx, bucket, object, uploadID, partID, data, data.Size(), putOpts)
+	info, err := l.Client.PutObjectPart(ctx, bucket, object, uploadID, partID, reader, data.Size(), putOpts)
 	if err != nil {
 		return pi, minio.ErrorRespToObjectError(err, bucket, object)
 	}
@@ -718,7 +758,7 @@ func (l *s3Objects) AbortMultipartUpload(ctx context.Context, bucket string, obj
 
 // CompleteMultipartUpload completes ongoing multipart upload and finalizes object
 func (l *s3Objects) CompleteMultipartUpload(ctx context.Context, bucket string, object string, uploadID string, uploadedParts []minio.CompletePart, opts minio.ObjectOptions) (oi minio.ObjectInfo, e error) {
-	uploadInfo, err := l.Client.CompleteMultipartUpload(ctx, bucket, object, uploadID, minio.ToMinioClientCompleteParts(uploadedParts), miniogo.PutObjectOptions{})
+	uploadInfo, err := l.Client.CompleteMultipartUpload(ctx, bucket, object, uploadID, minio.ToMinioClientCompleteParts(uploadedParts), miniogo.PutObjectOptions{SendContentMd5: true})
 	if err != nil {
 		return oi, minio.ErrorRespToObjectError(err, bucket, object)
 	}