diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7a8fbd35..d6fe7bfb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Added
 
-- Timeout option added to `Client.open` [#463](https://github.com/stac-utils/pystac-client/pull/463)
+- Timeout option added to `Client.open` [#463](https://github.com/stac-utils/pystac-client/pull/463), [#538](https://github.com/stac-utils/pystac-client/pull/538)
 - Support for fetching catalog queryables [#477](https://github.com/stac-utils/pystac-client/pull/477)
 - PySTAC Client specific warnings [#480](https://github.com/stac-utils/pystac-client/pull/480)
 - Support for fetching and merging a selection of queryables [#511](https://github.com/stac-utils/pystac-client/pull/511)
diff --git a/pystac_client/client.py b/pystac_client/client.py
index cec48861..4a967e06 100644
--- a/pystac_client/client.py
+++ b/pystac_client/client.py
@@ -101,7 +101,7 @@ def open(
         modifier: Optional[Callable[[Modifiable], None]] = None,
         request_modifier: Optional[Callable[[Request], Union[Request, None]]] = None,
         stac_io: Optional[StacApiIO] = None,
-        timeout: Timeout = None,
+        timeout: Optional[Timeout] = None,
     ) -> "Client":
         """Opens a STAC Catalog or API
         This function will read the root catalog of a STAC Catalog or API
@@ -197,7 +197,7 @@ def from_file(  # type: ignore
         parameters: Optional[Dict[str, Any]] = None,
         modifier: Optional[Callable[[Modifiable], None]] = None,
         request_modifier: Optional[Callable[[Request], Union[Request, None]]] = None,
-        timeout: Timeout = None,
+        timeout: Optional[Timeout] = None,
     ) -> "Client":
         """Open a STAC Catalog/API
 
diff --git a/pystac_client/stac_api_io.py b/pystac_client/stac_api_io.py
index b5592434..b0a1a802 100644
--- a/pystac_client/stac_api_io.py
+++ b/pystac_client/stac_api_io.py
@@ -39,7 +39,7 @@
 logger = logging.getLogger(__name__)
 
 
-Timeout: TypeAlias = Optional[Union[float, Tuple[float, float], Tuple[float, None]]]
+Timeout: TypeAlias = Union[float, Tuple[float, float], Tuple[float, None]]
 
 
 class StacApiIO(DefaultStacIO):
@@ -49,7 +49,7 @@ def __init__(
         conformance: Optional[List[str]] = None,
         parameters: Optional[Dict[str, Any]] = None,
         request_modifier: Optional[Callable[[Request], Union[Request, None]]] = None,
-        timeout: Timeout = None,
+        timeout: Optional[Timeout] = None,
         max_retries: Optional[int] = 5,
     ):
         """Initialize class for API IO
@@ -104,7 +104,7 @@ def update(
         headers: Optional[Dict[str, str]] = None,
         parameters: Optional[Dict[str, Any]] = None,
         request_modifier: Optional[Callable[[Request], Union[Request, None]]] = None,
-        timeout: Timeout = None,
+        timeout: Optional[Timeout] = None,
     ) -> None:
         """Updates this StacApi's headers, parameters, and/or request_modifer.