diff --git a/pre_commit/languages/python.py b/pre_commit/languages/python.py
index 65f521cd..e46089dd 100644
--- a/pre_commit/languages/python.py
+++ b/pre_commit/languages/python.py
@@ -196,7 +196,7 @@ def install_environment(
     python = norm_version(version)
     if python is not None:
         venv_cmd.extend(('-p', python))
-    install_cmd = ('python', '-mpip', 'install', '.', *additional_dependencies)
+    install_cmd = ('python', '-mpip', 'install', '--isolated', '.', *additional_dependencies)
 
     with clean_path_on_failure(envdir):
         cmd_output_b(*venv_cmd, cwd='/')