diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5bdaf6c95f..04840e386b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -399,6 +399,10 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed `parameters_to_ignore` not properly set to DDPWrapper ([#7239](https://github.com/PyTorchLightning/pytorch-lightning/pull/7239))
 
 
+- Fixed parsing of `fast_dev_run=True` with the built-in `ArgumentParser` ([#7240](https://github.com/PyTorchLightning/pytorch-lightning/pull/7240))
+
+
+
 ## [1.2.7] - 2021-04-06
 
 ### Fixed
diff --git a/pytorch_lightning/utilities/argparse.py b/pytorch_lightning/utilities/argparse.py
index dc99b923c6..9edc71997f 100644
--- a/pytorch_lightning/utilities/argparse.py
+++ b/pytorch_lightning/utilities/argparse.py
@@ -17,7 +17,7 @@ from argparse import _ArgumentGroup, ArgumentParser, Namespace
 from contextlib import suppress
 from typing import Any, Dict, List, Tuple, Union
 
-from pytorch_lightning.utilities.parsing import str_to_bool, str_to_bool_or_str
+from pytorch_lightning.utilities.parsing import str_to_bool, str_to_bool_or_str, str_to_bool_or_int
 
 
 def from_argparse_args(cls, args: Union[Namespace, ArgumentParser], **kwargs):
@@ -222,6 +222,8 @@ def add_argparse_args(
             # if the only arg type is bool
             if len(arg_types) == 1:
                 use_type = str_to_bool
+            elif int in arg_types:
+                use_type = str_to_bool_or_int
             elif str in arg_types:
                 use_type = str_to_bool_or_str
             else:
diff --git a/pytorch_lightning/utilities/parsing.py b/pytorch_lightning/utilities/parsing.py
index ae83ba15a9..37016a1293 100644
--- a/pytorch_lightning/utilities/parsing.py
+++ b/pytorch_lightning/utilities/parsing.py
@@ -56,6 +56,27 @@ def str_to_bool(val: str) -> bool:
     raise ValueError(f'invalid truth value {val}')
 
 
+def str_to_bool_or_int(val: str) -> Union[bool, int, str]:
+    """Convert a string representation to truth of bool if possible, or otherwise try to convert it to an int.
+
+    >>> str_to_bool_or_int("FALSE")
+    False
+    >>> str_to_bool_or_int("1")
+    True
+    >>> str_to_bool_or_int("2")
+    2
+    >>> str_to_bool_or_int("abc")
+    'abc'
+    """
+    val = str_to_bool_or_str(val)
+    if isinstance(val, bool):
+        return val
+    try:
+        return int(val)
+    except ValueError:
+        return val
+
+
 def is_picklable(obj: object) -> bool:
     """Tests if an object can be pickled"""
 
diff --git a/tests/trainer/test_trainer_cli.py b/tests/trainer/test_trainer_cli.py
index 7b91bcd941..a26883e897 100644
--- a/tests/trainer/test_trainer_cli.py
+++ b/tests/trainer/test_trainer_cli.py
@@ -175,6 +175,23 @@ def test_argparse_args_parsing(cli_args, expected):
     assert Trainer.from_argparse_args(args)
 
 
+@RunIf(min_python="3.7.0")
+@pytest.mark.parametrize('cli_args,expected', [
+    ('', False),
+    ('--fast_dev_run=0', False),
+    ('--fast_dev_run=True', True),
+    ('--fast_dev_run 2', 2),
+])
+def test_argparse_args_parsing_fast_dev_run(cli_args, expected):
+    """Test multi type argument with bool."""
+    cli_args = cli_args.split(' ') if cli_args else []
+    with mock.patch("argparse._sys.argv", ["any.py"] + cli_args):
+        parser = ArgumentParser(add_help=False)
+        parser = Trainer.add_argparse_args(parent_parser=parser)
+        args = Trainer.parse_argparser(parser)
+    assert args.fast_dev_run is expected
+
+
 @pytest.mark.parametrize(['cli_args', 'expected_parsed', 'expected_device_ids'], [
     pytest.param('', None, None),
     pytest.param('--gpus 1', 1, [0]),