diff --git a/src/urllib3/_async/connection.py b/src/urllib3/_async/connection.py
index 9f640d9cb0..760138bd14 100644
--- a/src/urllib3/_async/connection.py
+++ b/src/urllib3/_async/connection.py
@@ -424,6 +424,13 @@ async def request(
                 value = str(content_length)
             if enforce_charset_transparency and header.lower() == "content-type":
                 value_lower = value.lower()
+                # even if not "officially" supported
+                # some may send values as bytes, and we have to
+                # cast "temporarily" the value
+                # this case is already covered in the parent class.
+                if isinstance(value_lower, bytes):
+                    value_lower = value_lower.decode()
+                    value = value.decode()
                 if "charset" not in value_lower:
                     value = value.strip("; ")
                     value = f"{value}; charset=utf-8"
diff --git a/src/urllib3/connection.py b/src/urllib3/connection.py
index e607fff8c1..018a3bb86f 100644
--- a/src/urllib3/connection.py
+++ b/src/urllib3/connection.py
@@ -431,6 +431,13 @@ def request(
                 value = str(content_length)
             if enforce_charset_transparency and header.lower() == "content-type":
                 value_lower = value.lower()
+                # even if not "officially" supported
+                # some may send values as bytes, and we have to
+                # cast "temporarily" the value
+                # this case is already covered in the parent class.
+                if isinstance(value_lower, bytes):
+                    value_lower = value_lower.decode()
+                    value = value.decode()
                 if "charset" not in value_lower:
                     value = value.strip("; ")
                     value = f"{value}; charset=utf-8"
diff --git a/test/with_dummyserver/test_socketlevel.py b/test/with_dummyserver/test_socketlevel.py
index d5113dadc2..01d24d407c 100644
--- a/test/with_dummyserver/test_socketlevel.py
+++ b/test/with_dummyserver/test_socketlevel.py
@@ -2435,6 +2435,42 @@ def socket_handler(listener: socket.socket) -> None:
 
         assert b"Content-Type: application/json; charset=utf-8\r\n" in sent_bytes
 
+    def test_partial_overrule_bytes_content_type(self) -> None:
+        buffer = bytearray()
+
+        def socket_handler(listener: socket.socket) -> None:
+            nonlocal buffer
+            sock = listener.accept()[0]
+            sock.settimeout(0)
+
+            start = time.time()
+            while time.time() - start < (LONG_TIMEOUT / 2):
+                try:
+                    buffer += sock.recv(65536)
+                except OSError:
+                    continue
+
+            sock.sendall(
+                b"HTTP/1.1 200 OK\r\n"
+                b"Server: example.com\r\n"
+                b"Content-Length: 0\r\n\r\n"
+            )
+            sock.close()
+
+        self._start_server(socket_handler)
+
+        with HTTPConnectionPool(
+            self.host, self.port, timeout=LONG_TIMEOUT, retries=False
+        ) as pool:
+            resp = pool.request(
+                "POST", "/", body="{}", headers={"Content-Type": b"application/json"}  # type: ignore[dict-item]
+            )
+            assert resp.status == 200
+
+        sent_bytes = bytes(buffer)
+
+        assert b"Content-Type: application/json; charset=utf-8\r\n" in sent_bytes
+
     def test_no_overrule_str_content_type(self) -> None:
         buffer = bytearray()
 
diff --git a/test/with_traefik/asynchronous/test_send_data.py b/test/with_traefik/asynchronous/test_send_data.py
index 5d82949420..2f9fd791b6 100644
--- a/test/with_traefik/asynchronous/test_send_data.py
+++ b/test/with_traefik/asynchronous/test_send_data.py
@@ -28,6 +28,30 @@ async def test_overrule_unicode_content_length(self) -> None:
             assert "Content-Length" in (await resp.json())["headers"]
             assert (await resp.json())["headers"]["Content-Length"][0] == "4"
 
+    async def test_overrule_unicode_content_length_with_bytes_content_type(
+        self,
+    ) -> None:
+        async with AsyncHTTPSConnectionPool(
+            self.host,
+            self.https_port,
+            ca_certs=self.ca_authority,
+            resolver=self.test_async_resolver,
+        ) as p:
+            resp = await p.request(
+                "POST",
+                "/post",
+                body="🚀",
+                headers={"Content-Length": "1", "Content-Type": b"plain/text"},  # type: ignore[dict-item]
+            )
+
+            assert resp.status == 200
+            assert "Content-Length" in (await resp.json())["headers"]
+            assert "Content-Type" in (await resp.json())["headers"]
+            assert (await resp.json())["headers"]["Content-Type"][
+                0
+            ] == "plain/text; charset=utf-8"
+            assert (await resp.json())["headers"]["Content-Length"][0] == "4"
+
     @pytest.mark.parametrize(
         "method",
         [
diff --git a/test/with_traefik/test_send_data.py b/test/with_traefik/test_send_data.py
index 4526ad244e..e5141d91c8 100644
--- a/test/with_traefik/test_send_data.py
+++ b/test/with_traefik/test_send_data.py
@@ -25,6 +25,30 @@ def test_overrule_unicode_content_length(self) -> None:
             assert "Content-Length" in resp.json()["headers"]
             assert resp.json()["headers"]["Content-Length"][0] == "4"
 
+    def test_overrule_unicode_content_length_with_bytes_content_type(
+        self,
+    ) -> None:
+        with HTTPSConnectionPool(
+            self.host,
+            self.https_port,
+            ca_certs=self.ca_authority,
+            resolver=self.test_resolver,
+        ) as p:
+            resp = p.request(
+                "POST",
+                "/post",
+                body="🚀",
+                headers={"Content-Length": "1", "Content-Type": b"plain/text"},  # type: ignore[dict-item]
+            )
+
+            assert resp.status == 200
+            assert "Content-Length" in resp.json()["headers"]
+            assert "Content-Type" in resp.json()["headers"]
+            assert (
+                resp.json()["headers"]["Content-Type"][0] == "plain/text; charset=utf-8"
+            )
+            assert resp.json()["headers"]["Content-Length"][0] == "4"
+
     @pytest.mark.parametrize(
         "method",
         [