diff --git a/CHANGES/10074.feature.rst b/CHANGES/10074.feature.rst
new file mode 100644
index 00000000000..d956c38af57
--- /dev/null
+++ b/CHANGES/10074.feature.rst
@@ -0,0 +1,2 @@
+Added support for overriding the base URL with an absolute one in client sessions
+-- by :user:`vivodi`.
diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index 6adb3b97fb1..c3abc66bebf 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -52,6 +52,7 @@ Arseny Timoniq
 Artem Yushkovskiy
 Arthur Darcet
 Austin Scola
+Bai Haoran
 Ben Bader
 Ben Greiner
 Ben Kallus
diff --git a/aiohttp/client.py b/aiohttp/client.py
index e04a6ff989a..7539310aa8a 100644
--- a/aiohttp/client.py
+++ b/aiohttp/client.py
@@ -461,11 +461,9 @@ def request(
 
     def _build_url(self, str_or_url: StrOrURL) -> URL:
         url = URL(str_or_url)
-        if self._base_url is None:
-            return url
-        else:
-            assert not url.absolute
+        if self._base_url and not url.absolute:
             return self._base_url.join(url)
+        return url
 
     async def _request(
         self,
diff --git a/docs/client_reference.rst b/docs/client_reference.rst
index c9031de5383..7e7cdf12184 100644
--- a/docs/client_reference.rst
+++ b/docs/client_reference.rst
@@ -79,6 +79,10 @@ The client session supports the context manager protocol for self closing.
 
       .. versionadded:: 3.8
 
+      .. versionchanged:: 3.12
+
+         Added support for overriding the base URL with an absolute one in client sessions.
+
    :param aiohttp.BaseConnector connector: BaseConnector
       sub-class instance to support connection pooling.
 
diff --git a/tests/test_client_session.py b/tests/test_client_session.py
index 65f80b6abe9..a2c4833b83e 100644
--- a/tests/test_client_session.py
+++ b/tests/test_client_session.py
@@ -1086,6 +1086,24 @@ async def test_requote_redirect_setter() -> None:
             URL("http://example.com/test1/test2?q=foo#bar"),
             id="base_url=URL('http://example.com/test1/') url='test2?q=foo#bar'",
         ),
+        pytest.param(
+            URL("http://example.com/test1/"),
+            "http://foo.com/bar",
+            URL("http://foo.com/bar"),
+            id="base_url=URL('http://example.com/test1/') url='http://foo.com/bar'",
+        ),
+        pytest.param(
+            URL("http://example.com"),
+            "http://foo.com/bar",
+            URL("http://foo.com/bar"),
+            id="base_url=URL('http://example.com') url='http://foo.com/bar'",
+        ),
+        pytest.param(
+            URL("http://example.com/test1/"),
+            "http://foo.com",
+            URL("http://foo.com"),
+            id="base_url=URL('http://example.com/test1/') url='http://foo.com'",
+        ),
     ],
 )
 async def test_build_url_returns_expected_url(