diff --git a/changelogs/fragments/fix-build-files-manifest-walk.yml b/changelogs/fragments/fix-build-files-manifest-walk.yml
new file mode 100644
index 00000000000..3d1000698bf
--- /dev/null
+++ b/changelogs/fragments/fix-build-files-manifest-walk.yml
@@ -0,0 +1,4 @@
+minor_changes:
+  - >-
+    ansible-galaxy - Started normalizing build directory with a trailing separator when building collections, internally.
+    (https://github.com/ansible/ansible/pull/81619).
diff --git a/lib/ansible/galaxy/collection/__init__.py b/lib/ansible/galaxy/collection/__init__.py
index 60c9c94ba24..9bb9e941a47 100644
--- a/lib/ansible/galaxy/collection/__init__.py
+++ b/lib/ansible/galaxy/collection/__init__.py
@@ -1203,10 +1203,17 @@ def _build_files_manifest_walk(b_collection_path, namespace, name, ignore_patter
 
     manifest = _make_manifest()
 
+    def _discover_relative_base_directory(b_path: bytes, b_top_level_dir: bytes) -> bytes:
+        if b_path == b_top_level_dir:
+            return b''
+        common_prefix = os.path.commonpath((b_top_level_dir, b_path))
+        b_rel_base_dir = os.path.relpath(b_path, common_prefix)
+        return b_rel_base_dir.lstrip(os.path.sep.encode())
+
     def _walk(b_path, b_top_level_dir):
+        b_rel_base_dir = _discover_relative_base_directory(b_path, b_top_level_dir)
         for b_item in os.listdir(b_path):
             b_abs_path = os.path.join(b_path, b_item)
-            b_rel_base_dir = b'' if b_path == b_top_level_dir else b_path[len(b_top_level_dir) + 1:]
             b_rel_path = os.path.join(b_rel_base_dir, b_item)
             rel_path = to_text(b_rel_path, errors='surrogate_or_strict')
 
diff --git a/test/units/galaxy/test_collection.py b/test/units/galaxy/test_collection.py
index 991184aec1e..e8230d21073 100644
--- a/test/units/galaxy/test_collection.py
+++ b/test/units/galaxy/test_collection.py
@@ -39,10 +39,17 @@ def reset_cli_args():
     co.GlobalCLIArgs._Singleton__instance = None
 
 
-@pytest.fixture()
-def collection_input(tmp_path_factory):
-    ''' Creates a collection skeleton directory for build tests '''
-    test_dir = to_text(tmp_path_factory.mktemp('test-ÅÑŚÌβŁÈ Collections Input'))
+@pytest.fixture
+def collection_path_suffix(request):
+    """Return test collection path suffix or the default."""
+    return getattr(request, 'param', 'test-ÅÑŚÌβŁÈ Collections Input')
+
+
+@pytest.fixture
+def collection_input(tmp_path_factory, collection_path_suffix):
+    """Create a collection skeleton directory for build tests."""
+    test_dir = to_text(tmp_path_factory.mktemp(collection_path_suffix))
+
     namespace = 'ansible_namespace'
     collection = 'collection'
     skeleton = os.path.join(os.path.dirname(os.path.split(__file__)[0]), 'cli', 'test_data', 'collection_skeleton')
@@ -467,6 +474,14 @@ def test_build_existing_output_without_force(collection_input):
         collection.build_collection(to_text(input_dir, errors='surrogate_or_strict'), to_text(output_dir, errors='surrogate_or_strict'), False)
 
 
+@pytest.mark.parametrize(
+    'collection_path_suffix',
+    (
+        'test-ÅÑŚÌβŁÈ Collections Input 1 with_slash/',
+        'test-ÅÑŚÌβŁÈ Collections Input 2 no slash',
+    ),
+    indirect=('collection_path_suffix', ),
+)
 def test_build_existing_output_with_force(collection_input):
     input_dir, output_dir = collection_input