diff --git a/CHANGES.md b/CHANGES.md
index 00c210ba86f..00508eb9d32 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -48,6 +48,8 @@
 
 <!-- Changes that improve Black's performance. -->
 
+- Improve performance when a large directory is listed in `.gitignore` (#4415)
+
 ### Output
 
 <!-- Changes to Black's terminal output and error messages -->
diff --git a/src/black/files.py b/src/black/files.py
index c0cadbfd890..3be1ed9b00f 100644
--- a/src/black/files.py
+++ b/src/black/files.py
@@ -309,6 +309,8 @@ def _path_is_ignored(
     for gitignore_path, pattern in gitignore_dict.items():
         try:
             relative_path = path.relative_to(gitignore_path).as_posix()
+            if path.is_dir():
+                relative_path = relative_path + "/"
         except ValueError:
             break
         if pattern.match_file(relative_path):
diff --git a/tests/data/ignore_directory_gitignore_tests/.gitignore b/tests/data/ignore_directory_gitignore_tests/.gitignore
new file mode 100644
index 00000000000..4573ac5b3ac
--- /dev/null
+++ b/tests/data/ignore_directory_gitignore_tests/.gitignore
@@ -0,0 +1,3 @@
+large_ignored_dir/
+large_ignored_dir_two
+abc.py
diff --git a/tests/data/ignore_directory_gitignore_tests/abc.py b/tests/data/ignore_directory_gitignore_tests/abc.py
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/tests/data/ignore_directory_gitignore_tests/large_ignored_dir_two/a.py b/tests/data/ignore_directory_gitignore_tests/large_ignored_dir_two/a.py
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/tests/data/ignore_directory_gitignore_tests/large_ignored_dir_two/inner/b.py b/tests/data/ignore_directory_gitignore_tests/large_ignored_dir_two/inner/b.py
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/tests/data/ignore_directory_gitignore_tests/large_ignored_dir_two/inner2/c.py b/tests/data/ignore_directory_gitignore_tests/large_ignored_dir_two/inner2/c.py
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/tests/data/ignore_directory_gitignore_tests/large_ignored_dir_two/inner3/d.py b/tests/data/ignore_directory_gitignore_tests/large_ignored_dir_two/inner3/d.py
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/tests/data/ignore_directory_gitignore_tests/z.py b/tests/data/ignore_directory_gitignore_tests/z.py
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/tests/test_black.py b/tests/test_black.py
index 85141ae9124..34699b80663 100644
--- a/tests/test_black.py
+++ b/tests/test_black.py
@@ -2537,6 +2537,12 @@ def test_gitignore_that_ignores_subfolders(self) -> None:
         expected = [target / "b.py"]
         assert_collected_sources([target], expected, root=root)
 
+    def test_gitignore_that_ignores_directory(self) -> None:
+        # If gitignore with a directory is in root
+        root = Path(DATA_DIR, "ignore_directory_gitignore_tests")
+        expected = [root / "z.py"]
+        assert_collected_sources([root], expected, root=root)
+
     def test_empty_include(self) -> None:
         path = DATA_DIR / "include_exclude_tests"
         src = [path]