2 files changed, 9 insertions, 1 deletions
diff --git a/pre_commit/commands/hook_impl.py b/pre_commit/commands/hook_impl.py
index 25c5fdf..a766ee9 100644
--- a/pre_commit/commands/hook_impl.py
+++ b/pre_commit/commands/hook_impl.py
@@ -76,6 +76,7 @@ def _ns(
         remote_url: Optional[str] = None,
         commit_msg_filename: Optional[str] = None,
         checkout_type: Optional[str] = None,
+        is_squash_merge: Optional[str] = None,
 ) -> argparse.Namespace:
     return argparse.Namespace(
         color=color,
@@ -88,6 +89,7 @@ def _ns(
         commit_msg_filename=commit_msg_filename,
         all_files=all_files,
         checkout_type=checkout_type,
+        is_squash_merge=is_squash_merge,
         files=(),
         hook=None,
         verbose=False,
@@ -158,6 +160,7 @@ _EXPECTED_ARG_LENGTH_BY_HOOK = {
     'post-commit': 0,
     'pre-commit': 0,
     'pre-merge-commit': 0,
+    'post-merge': 1,
     'pre-push': 2,
 }
 
@@ -199,6 +202,8 @@ def _run_ns(
             hook_type, color,
             from_ref=args[0], to_ref=args[1], checkout_type=args[2],
         )
+    elif hook_type == 'post-merge':
+        return _ns(hook_type, color, is_squash_merge=args[0])
     else:
         raise AssertionError(f'unexpected hook type: {hook_type}')
 
diff --git a/pre_commit/commands/run.py b/pre_commit/commands/run.py
index 891488d..05c3268 100644
--- a/pre_commit/commands/run.py
+++ b/pre_commit/commands/run.py
@@ -245,7 +245,7 @@ def _compute_cols(hooks: Sequence[Hook]) -> int:
 
 def _all_filenames(args: argparse.Namespace) -> Collection[str]:
     # these hooks do not operate on files
-    if args.hook_stage in {'post-checkout', 'post-commit'}:
+    if args.hook_stage in {'post-checkout', 'post-commit', 'post-merge'}:
         return ()
     elif args.hook_stage in {'prepare-commit-msg', 'commit-msg'}:
         return (args.commit_msg_filename,)
@@ -379,6 +379,9 @@ def run(
     if args.checkout_type:
         environ['PRE_COMMIT_CHECKOUT_TYPE'] = args.checkout_type
 
+    if args.is_squash_merge:
+        environ['PRE_COMMIT_IS_SQUASH_MERGE'] = args.is_squash_merge
+
     # Set pre_commit flag
     environ['PRE_COMMIT'] = '1'