diff --git a/changelog.d/9069.misc b/changelog.d/9069.misc
new file mode 100644
index 0000000000..5e9e62d252
--- /dev/null
+++ b/changelog.d/9069.misc
@@ -0,0 +1 @@
+Remove `SynapseRequest.get_user_agent`.
diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index 48c4d7b0be..6d6703250b 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -33,6 +33,7 @@ from synapse.api.errors import (
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
 from synapse.appservice import ApplicationService
 from synapse.events import EventBase
+from synapse.http import get_request_user_agent
 from synapse.http.site import SynapseRequest
 from synapse.logging import opentracing as opentracing
 from synapse.storage.databases.main.registration import TokenLookupResult
@@ -187,7 +188,7 @@ class Auth:
         """
         try:
             ip_addr = self.hs.get_ip_from_request(request)
-            user_agent = request.get_user_agent("")
+            user_agent = get_request_user_agent(request)
 
             access_token = self.get_access_token_from_request(request)
 
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index f4434673dc..5b86ee85c7 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -49,8 +49,10 @@ from synapse.api.errors import (
     UserDeactivatedError,
 )
 from synapse.api.ratelimiting import Ratelimiter
+from synapse.handlers._base import BaseHandler
 from synapse.handlers.ui_auth import INTERACTIVE_AUTH_CHECKERS
 from synapse.handlers.ui_auth.checkers import UserInteractiveAuthChecker
+from synapse.http import get_request_user_agent
 from synapse.http.server import finish_request, respond_with_html
 from synapse.http.site import SynapseRequest
 from synapse.logging.context import defer_to_thread
@@ -62,8 +64,6 @@ from synapse.util.async_helpers import maybe_awaitable
 from synapse.util.msisdn import phone_number_to_msisdn
 from synapse.util.threepids import canonicalise_email
 
-from ._base import BaseHandler
-
 if TYPE_CHECKING:
     from synapse.app.homeserver import HomeServer
 
@@ -539,7 +539,7 @@ class AuthHandler(BaseHandler):
             # authentication flow.
             await self.store.set_ui_auth_clientdict(sid, clientdict)
 
-        user_agent = request.get_user_agent("")
+        user_agent = get_request_user_agent(request)
 
         await self.store.add_user_agent_ip_to_ui_auth_session(
             session.session_id, user_agent, clientip
diff --git a/synapse/handlers/sso.py b/synapse/handlers/sso.py
index 2da1ea2223..740df7e4a0 100644
--- a/synapse/handlers/sso.py
+++ b/synapse/handlers/sso.py
@@ -23,6 +23,7 @@ from typing_extensions import NoReturn, Protocol
 from twisted.web.http import Request
 
 from synapse.api.errors import Codes, RedirectException, SynapseError
+from synapse.http import get_request_user_agent
 from synapse.http.server import respond_with_html
 from synapse.http.site import SynapseRequest
 from synapse.types import JsonDict, UserID, contains_invalid_mxid_characters
@@ -362,7 +363,7 @@ class SsoHandler:
                     attributes,
                     auth_provider_id,
                     remote_user_id,
-                    request.get_user_agent(""),
+                    get_request_user_agent(request),
                     request.getClientIP(),
                 )
 
@@ -628,7 +629,7 @@ class SsoHandler:
             attributes,
             session.auth_provider_id,
             session.remote_user_id,
-            request.get_user_agent(""),
+            get_request_user_agent(request),
             request.getClientIP(),
         )
 
diff --git a/synapse/http/__init__.py b/synapse/http/__init__.py
index 59b01b812c..4bc3cb53f0 100644
--- a/synapse/http/__init__.py
+++ b/synapse/http/__init__.py
@@ -17,6 +17,7 @@ import re
 
 from twisted.internet import task
 from twisted.web.client import FileBodyProducer
+from twisted.web.iweb import IRequest
 
 from synapse.api.errors import SynapseError
 
@@ -50,3 +51,17 @@ class QuieterFileBodyProducer(FileBodyProducer):
             FileBodyProducer.stopProducing(self)
         except task.TaskStopped:
             pass
+
+
+def get_request_user_agent(request: IRequest, default: str = "") -> str:
+    """Return the last User-Agent header, or the given default.
+    """
+    # There could be raw utf-8 bytes in the User-Agent header.
+
+    # N.B. if you don't do this, the logger explodes cryptically
+    # with maximum recursion trying to log errors about
+    # the charset problem.
+    # c.f. https://github.com/matrix-org/synapse/issues/3471
+
+    h = request.getHeader(b"User-Agent")
+    return h.decode("ascii", "replace") if h else default
diff --git a/synapse/http/site.py b/synapse/http/site.py
index 5a5790831b..12ec3f851f 100644
--- a/synapse/http/site.py
+++ b/synapse/http/site.py
@@ -20,7 +20,7 @@ from twisted.python.failure import Failure
 from twisted.web.server import Request, Site
 
 from synapse.config.server import ListenerConfig
-from synapse.http import redact_uri
+from synapse.http import get_request_user_agent, redact_uri
 from synapse.http.request_metrics import RequestMetrics, requests_counter
 from synapse.logging.context import LoggingContext, PreserveLoggingContext
 from synapse.types import Requester
@@ -113,15 +113,6 @@ class SynapseRequest(Request):
             method = self.method.decode("ascii")
         return method
 
-    def get_user_agent(self, default: str) -> str:
-        """Return the last User-Agent header, or the given default.
-        """
-        user_agent = self.requestHeaders.getRawHeaders(b"User-Agent", [None])[-1]
-        if user_agent is None:
-            return default
-
-        return user_agent.decode("ascii", "replace")
-
     def render(self, resrc):
         # this is called once a Resource has been found to serve the request; in our
         # case the Resource in question will normally be a JsonResource.
@@ -292,12 +283,7 @@ class SynapseRequest(Request):
             # and can see that we're doing something wrong.
             authenticated_entity = repr(self.requester)  # type: ignore[unreachable]
 
-        # ...or could be raw utf-8 bytes in the User-Agent header.
-        # N.B. if you don't do this, the logger explodes cryptically
-        # with maximum recursion trying to log errors about
-        # the charset problem.
-        # c.f. https://github.com/matrix-org/synapse/issues/3471
-        user_agent = self.get_user_agent("-")
+        user_agent = get_request_user_agent(self, "-")
 
         code = str(self.code)
         if not self.finished:
diff --git a/tests/handlers/test_cas.py b/tests/handlers/test_cas.py
index bd7a1b6891..c37bb6440e 100644
--- a/tests/handlers/test_cas.py
+++ b/tests/handlers/test_cas.py
@@ -118,4 +118,4 @@ class CasHandlerTestCase(HomeserverTestCase):
 
 def _mock_request():
     """Returns a mock which will stand in as a SynapseRequest"""
-    return Mock(spec=["getClientIP", "get_user_agent"])
+    return Mock(spec=["getClientIP", "getHeader"])
diff --git a/tests/handlers/test_oidc.py b/tests/handlers/test_oidc.py
index f5df657814..4ce0f74f22 100644
--- a/tests/handlers/test_oidc.py
+++ b/tests/handlers/test_oidc.py
@@ -1011,7 +1011,7 @@ def _build_callback_request(
             "addCookie",
             "requestHeaders",
             "getClientIP",
-            "get_user_agent",
+            "getHeader",
         ]
     )
 
@@ -1020,5 +1020,4 @@ def _build_callback_request(
     request.args[b"code"] = [code.encode("utf-8")]
     request.args[b"state"] = [state.encode("utf-8")]
     request.getClientIP.return_value = ip_address
-    request.get_user_agent.return_value = user_agent
     return request
diff --git a/tests/handlers/test_saml.py b/tests/handlers/test_saml.py
index 548038214b..261c7083d1 100644
--- a/tests/handlers/test_saml.py
+++ b/tests/handlers/test_saml.py
@@ -262,4 +262,4 @@ class SamlHandlerTestCase(HomeserverTestCase):
 
 def _mock_request():
     """Returns a mock which will stand in as a SynapseRequest"""
-    return Mock(spec=["getClientIP", "get_user_agent"])
+    return Mock(spec=["getClientIP", "getHeader"])