diff --git a/storage/google/cloud/storage/bucket.py b/storage/google/cloud/storage/bucket.py
index 48ab09e23e4f..eabf883455ad 100644
--- a/storage/google/cloud/storage/bucket.py
+++ b/storage/google/cloud/storage/bucket.py
@@ -1064,7 +1064,7 @@ def copy_blob(
         destination_bucket,
         new_name=None,
         client=None,
-        preserve_acl=True,
+        preserve_acl=None,
         source_generation=None,
     ):
         """Copy the given blob to the given bucket, optionally with a new name.
@@ -1088,7 +1088,9 @@ def copy_blob(
 
         :type preserve_acl: bool
         :param preserve_acl: Optional. Copies ACL from old blob to new blob.
-                             Default: True.
+                             Default: None. Deprecated: this argument is no longer
+                             affecting anything and will be removed in a future release.
+                             Please update the blob's ACL manually.
 
         :type source_generation: long
         :param source_generation: Optional. The generation of the blob to be
@@ -1118,7 +1120,14 @@ def copy_blob(
             _target_object=new_blob,
         )
 
-        if not preserve_acl:
+        if preserve_acl is not None:
+            warnings.warn(
+                "The 'preserve_acl' argument is deprecated. For forward compatibility, "
+                "please update the blob's ACL manually.",
+                DeprecationWarning,
+                stacklevel=2,
+            )
+        if not preserve_acl and preserve_acl is not None:
             new_blob.acl.save(acl={}, client=client)
 
         new_blob._set_properties(copy_result)
diff --git a/storage/tests/unit/test_bucket.py b/storage/tests/unit/test_bucket.py
index de943339e200..52e9d38a1d80 100644
--- a/storage/tests/unit/test_bucket.py
+++ b/storage/tests/unit/test_bucket.py
@@ -1152,7 +1152,8 @@ def test_copy_blobs_source_generation(self):
         self.assertEqual(kw["path"], COPY_PATH)
         self.assertEqual(kw["query_params"], {"sourceGeneration": GENERATION})
 
-    def test_copy_blobs_preserve_acl(self):
+    @mock.patch("warnings.warn")
+    def test_copy_blobs_preserve_acl_deprecated(self, mock_warn):
         from google.cloud.storage.acl import ObjectACL
 
         SOURCE = "source"
@@ -1170,6 +1171,13 @@ def test_copy_blobs_preserve_acl(self):
             blob, dest, NEW_NAME, client=client, preserve_acl=False
         )
 
+        mock_warn.assert_called_with(
+            "The 'preserve_acl' argument is deprecated. For forward compatibility, "
+            "please update the blob's ACL manually.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+
         self.assertIs(new_blob.bucket, dest)
         self.assertEqual(new_blob.name, NEW_NAME)
         self.assertIsInstance(new_blob.acl, ObjectACL)