diff --git a/pre_commit/commands/run.py b/pre_commit/commands/run.py
index c2dab6f7..f6916c26 100644
--- a/pre_commit/commands/run.py
+++ b/pre_commit/commands/run.py
@@ -347,6 +347,9 @@ def run(
     if args.checkout_type:
         environ['PRE_COMMIT_CHECKOUT_TYPE'] = args.checkout_type
 
+    # Set pre_commit flag
+    environ['PRE_COMMIT'] = '1'
+
     with contextlib.ExitStack() as exit_stack:
         if stash:
             exit_stack.enter_context(staged_files_only(store.directory))
diff --git a/tests/commands/run_test.py b/tests/commands/run_test.py
index 2fffdb91..cf9794ed 100644
--- a/tests/commands/run_test.py
+++ b/tests/commands/run_test.py
@@ -1028,3 +1028,12 @@ def test_skipped_without_any_setup_for_post_checkout(in_git_dir, store):
     environ = {'_PRE_COMMIT_SKIP_POST_CHECKOUT': '1'}
     opts = run_opts(hook_stage='post-checkout')
     assert run(C.CONFIG_FILE, store, opts, environ=environ) == 0
+
+
+def test_pre_commit_env_variable_set(cap_out, store, repo_with_passing_hook):
+    args = run_opts()
+    environ: EnvironT = {}
+    ret, printed = _do_run(
+        cap_out, store, repo_with_passing_hook, args, environ,
+    )
+    assert environ['PRE_COMMIT'] == '1'