diff --git a/google/cloud/firestore_v1/async_transaction.py b/google/cloud/firestore_v1/async_transaction.py
index be8668cd6..36509941e 100644
--- a/google/cloud/firestore_v1/async_transaction.py
+++ b/google/cloud/firestore_v1/async_transaction.py
@@ -15,7 +15,14 @@
 """Helpers for applying Google Cloud Firestore changes in a transaction."""
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Any, AsyncGenerator, Callable, Coroutine, Optional
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    AsyncGenerator,
+    Awaitable,
+    Callable,
+    Optional,
+)
 
 from google.api_core import exceptions, gapic_v1
 from google.api_core import retry_async as retries
@@ -37,11 +44,15 @@
 # Types needed only for Type Hints
 if TYPE_CHECKING:  # pragma: NO COVER
     import datetime
+    from typing_extensions import TypeVar, ParamSpec, Concatenate
 
     from google.cloud.firestore_v1.async_stream_generator import AsyncStreamGenerator
     from google.cloud.firestore_v1.base_document import DocumentSnapshot
     from google.cloud.firestore_v1.query_profile import ExplainOptions
 
+    T = TypeVar("T")
+    P = ParamSpec("P")
+
 
 class AsyncTransaction(async_batch.AsyncWriteBatch, BaseTransaction):
     """Accumulate read-and-write operations to be sent in a transaction.
@@ -253,12 +264,14 @@ class _AsyncTransactional(_BaseTransactional):
             A coroutine that should be run (and retried) in a transaction.
     """
 
-    def __init__(self, to_wrap) -> None:
+    def __init__(
+        self, to_wrap: Callable[Concatenate[AsyncTransaction, P], Awaitable[T]]
+    ) -> None:
         super(_AsyncTransactional, self).__init__(to_wrap)
 
     async def _pre_commit(
-        self, transaction: AsyncTransaction, *args, **kwargs
-    ) -> Coroutine:
+        self, transaction: AsyncTransaction, *args: P.args, **kwargs: P.kwargs
+    ) -> T:
         """Begin transaction and call the wrapped coroutine.
 
         Args:
@@ -271,7 +284,7 @@ async def _pre_commit(
                 along to the wrapped coroutine.
 
         Returns:
-            Any: result of the wrapped coroutine.
+            T: result of the wrapped coroutine.
 
         Raises:
             Exception: Any failure caused by ``to_wrap``.
@@ -286,12 +299,14 @@ async def _pre_commit(
             self.retry_id = self.current_id
         return await self.to_wrap(transaction, *args, **kwargs)
 
-    async def __call__(self, transaction, *args, **kwargs):
+    async def __call__(
+        self, transaction: AsyncTransaction, *args: P.args, **kwargs: P.kwargs
+    ) -> T:
         """Execute the wrapped callable within a transaction.
 
         Args:
             transaction
-                (:class:`~google.cloud.firestore_v1.transaction.Transaction`):
+                (:class:`~google.cloud.firestore_v1.async_transaction.AsyncTransaction`):
                 A transaction to execute the callable within.
             args (Tuple[Any, ...]): The extra positional arguments to pass
                 along to the wrapped callable.
@@ -299,7 +314,7 @@ async def __call__(self, transaction, *args, **kwargs):
                 along to the wrapped callable.
 
         Returns:
-            Any: The result of the wrapped callable.
+            T: The result of the wrapped callable.
 
         Raises:
             ValueError: If the transaction does not succeed in
@@ -313,7 +328,7 @@ async def __call__(self, transaction, *args, **kwargs):
 
         try:
             for attempt in range(transaction._max_attempts):
-                result = await self._pre_commit(transaction, *args, **kwargs)
+                result: T = await self._pre_commit(transaction, *args, **kwargs)
                 try:
                     await transaction._commit()
                     return result
@@ -338,17 +353,17 @@ async def __call__(self, transaction, *args, **kwargs):
 
 
 def async_transactional(
-    to_wrap: Callable[[AsyncTransaction], Any]
-) -> _AsyncTransactional:
+    to_wrap: Callable[Concatenate[AsyncTransaction, P], Awaitable[T]]
+) -> Callable[Concatenate[AsyncTransaction, P], Awaitable[T]]:
     """Decorate a callable so that it runs in a transaction.
 
     Args:
         to_wrap
-            (Callable[[:class:`~google.cloud.firestore_v1.transaction.Transaction`, ...], Any]):
+            (Callable[[:class:`~google.cloud.firestore_v1.async_transaction.AsyncTransaction`, ...], Awaitable[Any]]):
             A callable that should be run (and retried) in a transaction.
 
     Returns:
-        Callable[[:class:`~google.cloud.firestore_v1.transaction.Transaction`, ...], Any]:
+        Callable[[:class:`~google.cloud.firestore_v1.transaction.Transaction`, ...], Awaitable[Any]]:
         the wrapped callable.
     """
     return _AsyncTransactional(to_wrap)
diff --git a/mypy.ini b/mypy.ini
index 4505b4854..beaa679a8 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -1,3 +1,3 @@
 [mypy]
-python_version = 3.6
+python_version = 3.8
 namespace_packages = True