diff --git a/pre_commit/commands.py b/pre_commit/commands.py
index ac9d4487..33e36879 100644
--- a/pre_commit/commands.py
+++ b/pre_commit/commands.py
@@ -229,11 +229,21 @@ def _run_hook(runner, hook_id, args, write):
         return 1
 
 
+def _has_unmerged_paths(runner):
+    _, stdout, _ = runner.cmd_runner.run(['git', 'ls-files', '--unmerged'])
+    return bool(stdout.strip())
+
+
 def run(runner, args, write=sys.stdout.write):
     # Set up our logging handler
     logger.addHandler(LoggingHandler(args.color, write=write))
     logger.setLevel(logging.INFO)
 
+    # Check if we have unresolved merge conflict files and fail fast.
+    if _has_unmerged_paths(runner):
+        logger.error('Unmerged files.  Resolve before committing.')
+        return 1
+
     if args.no_stash or args.all_files:
         ctx = noop_context()
     else:
diff --git a/tests/commands_test.py b/tests/commands_test.py
index ae4e3b4c..e3e6eb17 100644
--- a/tests/commands_test.py
+++ b/tests/commands_test.py
@@ -275,3 +275,47 @@ def test_no_stash(repo_with_passing_hook, no_stash, all_files, expect_stash):
         assert warning_msg in printed
     else:
         assert warning_msg not in printed
+
+
+@pytest.mark.parametrize(('output', 'expected'), (('some', True), ('', False)))
+def test_has_unmerged_paths(output, expected):
+    mock_runner = mock.Mock()
+    mock_runner.cmd_runner.run.return_value = (1, output, '')
+    assert commands._has_unmerged_paths(mock_runner) is expected
+
+
+@pytest.yield_fixture
+def in_merge_conflict(repo_with_passing_hook):
+    local['git']['add', C.CONFIG_FILE]()
+    local['git']['commit', '-m' 'add hooks file']()
+    local['git']['clone', '.', 'foo']()
+    with local.cwd('foo'):
+        local['git']['checkout', 'origin/master', '-b', 'foo']()
+        with open('conflict_file', 'w') as conflict_file:
+            conflict_file.write('herp\nderp\n')
+        local['git']['add', 'conflict_file']()
+        local['git']['commit', '-m', 'conflict_file']()
+        local['git']['checkout', 'origin/master', '-b', 'bar']()
+        with open('conflict_file', 'w') as conflict_file:
+            conflict_file.write('harp\nddrp\n')
+        local['git']['add', 'conflict_file']()
+        local['git']['commit', '-m', 'conflict_file']()
+        local['git']['merge', 'foo'](retcode=None)
+        yield os.path.join(repo_with_passing_hook, 'foo')
+
+
+def test_merge_conflict(in_merge_conflict):
+    ret, printed = _do_run(in_merge_conflict, _get_opts())
+    assert ret == 1
+    assert 'Unmerged files.  Resolve before committing.' in printed
+
+
+def test_merge_conflict_modified(in_merge_conflict):
+    # Touch another file so we have unstaged non-conflicting things
+    assert os.path.exists('dummy')
+    with open('dummy', 'w') as dummy_file:
+        dummy_file.write('bar\nbaz\n')
+
+    ret, printed = _do_run(in_merge_conflict, _get_opts())
+    assert ret == 1
+    assert 'Unmerged files.  Resolve before committing.' in printed
diff --git a/tests/staged_files_only_test.py b/tests/staged_files_only_test.py
index adb0b015..e3533733 100644
--- a/tests/staged_files_only_test.py
+++ b/tests/staged_files_only_test.py
@@ -227,7 +227,7 @@ def fake_logging_handler():
             self.logs = []
 
         def emit(self, record):
-            self.logs.append(record)
+            self.logs.append(record)  # pragma: no cover (only hit in failure)
 
     pre_commit_logger = logging.getLogger('pre_commit')
     original_level = pre_commit_logger.getEffectiveLevel()