diff --git a/pkg/block/gs/adapter.go b/pkg/block/gs/adapter.go
index 4ae38b67df8..43328cddeb4 100644
--- a/pkg/block/gs/adapter.go
+++ b/pkg/block/gs/adapter.go
@@ -7,6 +7,7 @@ import (
 	"io"
 	"net/http"
 	"net/url"
+	"sort"
 	"strings"
 	"time"
 
@@ -25,7 +26,10 @@ const (
 )
 
 var (
+	ErrMismatchPartETag    = errors.New("mismatch part ETag")
+	ErrMismatchPartName    = errors.New("mismatch part name")
 	ErrMaxMultipartObjects = errors.New("maximum multipart object reached")
+	ErrPartListMismatch    = errors.New("multipart part list mismatch")
 	ErrMissingTargetAttrs  = errors.New("missing target attributes")
 )
 
@@ -499,7 +503,7 @@ func (a *Adapter) CompleteMultiPartUpload(ctx context.Context, obj block.ObjectP
 	})
 	lg.Debug("started multipart upload")
 
-	parts, err := a.getPartNames(bucketName, uploadID, multipartList)
+	parts, err := a.getPartNamesWithValidation(ctx, bucketName, uploadID, multipartList)
 	if err != nil {
 		return nil, err
 	}
@@ -524,18 +528,89 @@ func (a *Adapter) CompleteMultiPartUpload(ctx context.Context, obj block.ObjectP
 	}, nil
 }
 
-func (a *Adapter) getPartNames(bucketName, uploadID string, multipartList *block.MultipartUploadCompletion) ([]string, error) {
-	if len(multipartList.Part) > MaxMultipartObjects {
-		return nil, fmt.Errorf("listing bucket '%s' upload '%s': %w", bucketName, uploadID, ErrMaxMultipartObjects)
+func (a *Adapter) getPartNamesWithValidation(ctx context.Context, bucketName, uploadID string, multipartList *block.MultipartUploadCompletion) ([]string, error) {
+	// list bucket parts and validate request match
+	bucketParts, err := a.listMultipartUploadParts(ctx, bucketName, uploadID)
+	if err != nil {
+		return nil, err
+	}
+	// validate bucketParts match the request multipartList
+	err = a.validateMultipartUploadParts(uploadID, multipartList, bucketParts)
+	if err != nil {
+		return nil, err
 	}
 
-	parts := make([]string, len(multipartList.Part))
-	for i := 0; i < len(parts); i++ {
-		parts[i] = formatMultipartFilename(uploadID, i+1)
+	// prepare names
+	parts := make([]string, len(bucketParts))
+	for i, part := range bucketParts {
+		parts[i] = part.Name
 	}
 	return parts, nil
 }
 
+func (a *Adapter) validateMultipartUploadParts(uploadID string, multipartList *block.MultipartUploadCompletion, bucketParts []*storage.ObjectAttrs) error {
+	if len(multipartList.Part) != len(bucketParts) {
+		return fmt.Errorf("part list mismatch - expected %d parts, got %d: %w", len(bucketParts), len(multipartList.Part), ErrPartListMismatch)
+	}
+	for i, p := range multipartList.Part {
+		objName := formatMultipartFilename(uploadID, p.PartNumber)
+		if objName != bucketParts[i].Name {
+			return fmt.Errorf("invalid part at position %d: %w", i, ErrMismatchPartName)
+		}
+		if p.ETag != bucketParts[i].Etag {
+			return fmt.Errorf("invalid part at position %d: %w", i, ErrMismatchPartETag)
+		}
+	}
+	return nil
+}
+
+func (a *Adapter) listMultipartUploadParts(ctx context.Context, bucketName string, uploadID string) ([]*storage.ObjectAttrs, error) {
+	bucket := a.client.Bucket(bucketName)
+	var bucketParts []*storage.ObjectAttrs
+	query := &storage.Query{
+		Delimiter: delimiter,
+		Prefix:    uploadID + partSuffix,
+	}
+	err := query.SetAttrSelection([]string{"Name", "Etag"})
+	if err != nil {
+		return nil, err
+	}
+	it := bucket.Objects(ctx, query)
+	for {
+		attrs, err := it.Next()
+		if errors.Is(err, iterator.Done) {
+			break
+		}
+		if err != nil {
+			return nil, fmt.Errorf("listing bucket '%s' upload '%s': %w", bucketName, uploadID, err)
+		}
+
+		// filter out invalid part names
+		if !a.isPartName(attrs.Name) {
+			continue
+		}
+
+		bucketParts = append(bucketParts, attrs)
+		if len(bucketParts) > MaxMultipartObjects {
+			return nil, fmt.Errorf("listing bucket '%s' upload '%s': %w", bucketName, uploadID, ErrMaxMultipartObjects)
+		}
+	}
+	// sort by name - assume natual sort order
+	sort.Slice(bucketParts, func(i, j int) bool {
+		return bucketParts[i].Name < bucketParts[j].Name
+	})
+	return bucketParts, nil
+}
+
+// isPartName checks it's a valid part name, as opposed to an already merged group of parts
+func (a *Adapter) isPartName(name string) bool {
+	if len(name) < len(partSuffix)+5 {
+		return false
+	}
+	suffixSubstring := name[len(name)-5-len(partSuffix) : len(name)-5]
+	return partSuffix == suffixSubstring
+}
+
 func (a *Adapter) composeMultipartUploadParts(ctx context.Context, bucketName string, uploadID string, parts []string) (*storage.ObjectAttrs, error) {
 	// compose target from all parts
 	bucket := a.client.Bucket(bucketName)