diff --git a/moto/s3/models.py b/moto/s3/models.py
index e6365dabf46f..930d7625697e 100644
--- a/moto/s3/models.py
+++ b/moto/s3/models.py
@@ -1888,9 +1888,13 @@ def put_object(
         return new_key
 
     def put_object_acl(
-        self, bucket_name: str, key_name: str, acl: Optional[FakeAcl]
+        self,
+        bucket_name: str,
+        key_name: str,
+        acl: Optional[FakeAcl],
+        key_is_clean: bool = False,
     ) -> None:
-        key = self.get_object(bucket_name, key_name)
+        key = self.get_object(bucket_name, key_name, key_is_clean=key_is_clean)
         # TODO: Support the XML-based ACL format
         if key is not None:
             key.set_acl(acl)
diff --git a/moto/s3/responses.py b/moto/s3/responses.py
index 10270de8ffa2..da42bac69880 100644
--- a/moto/s3/responses.py
+++ b/moto/s3/responses.py
@@ -2173,7 +2173,12 @@ def _key_response_post(
             )
             key.set_metadata(multipart.metadata)
             self.backend.set_key_tags(key, multipart.tags)
-            self.backend.put_object_acl(bucket_name, key.name, multipart.acl)
+            self.backend.put_object_acl(
+                bucket_name=bucket_name,
+                key_name=key.name,
+                acl=multipart.acl,
+                key_is_clean=True,
+            )
 
             template = self.response_template(S3_MULTIPART_COMPLETE_RESPONSE)
             headers: Dict[str, Any] = {}
diff --git a/tests/test_s3/test_s3_multipart.py b/tests/test_s3/test_s3_multipart.py
index b152c88bad0c..fb511e76afa5 100644
--- a/tests/test_s3/test_s3_multipart.py
+++ b/tests/test_s3/test_s3_multipart.py
@@ -107,34 +107,35 @@ def test_multipart_upload_too_small():
     )
 
 
+@pytest.mark.parametrize("key", ["the-key", "the%20key"])
 @mock_s3
 @reduced_min_part_size
-def test_multipart_upload():
+def test_multipart_upload(key: str):
     s3 = boto3.resource("s3", region_name=DEFAULT_REGION_NAME)
     client = boto3.client("s3", region_name=DEFAULT_REGION_NAME)
     s3.create_bucket(Bucket="foobar")
 
     part1 = b"0" * REDUCED_PART_SIZE
     part2 = b"1"
-    mp = client.create_multipart_upload(Bucket="foobar", Key="the-key")
+    mp = client.create_multipart_upload(Bucket="foobar", Key=key)
     up1 = client.upload_part(
         Body=BytesIO(part1),
         PartNumber=1,
         Bucket="foobar",
-        Key="the-key",
+        Key=key,
         UploadId=mp["UploadId"],
     )
     up2 = client.upload_part(
         Body=BytesIO(part2),
         PartNumber=2,
         Bucket="foobar",
-        Key="the-key",
+        Key=key,
         UploadId=mp["UploadId"],
     )
 
     client.complete_multipart_upload(
         Bucket="foobar",
-        Key="the-key",
+        Key=key,
         MultipartUpload={
             "Parts": [
                 {"ETag": up1["ETag"], "PartNumber": 1},
@@ -144,7 +145,7 @@ def test_multipart_upload():
         UploadId=mp["UploadId"],
     )
     # we should get both parts as the key contents
-    response = client.get_object(Bucket="foobar", Key="the-key")
+    response = client.get_object(Bucket="foobar", Key=key)
     response["Body"].read().should.equal(part1 + part2)