diff --git a/changelog.d/19949.bugfix b/changelog.d/19949.bugfix
new file mode 100644
index 0000000000..24ddd4dc4f
--- /dev/null
+++ b/changelog.d/19949.bugfix
@@ -0,0 +1 @@
+Fix `SYNAPSE_ASYNC_IO_REACTOR=1` on Python 3.14.
diff --git a/synapse/__init__.py b/synapse/__init__.py
index 3acfc1a0d7..a223066f04 100644
--- a/synapse/__init__.py
+++ b/synapse/__init__.py
@@ -49,7 +49,9 @@ if strtobool(os.environ.get("SYNAPSE_ASYNC_IO_REACTOR", "0")):
 
     from twisted.internet import asyncioreactor
 
-    asyncioreactor.install(asyncio.get_event_loop())
+    loop = asyncio.new_event_loop()
+    asyncio.set_event_loop(loop)
+    asyncioreactor.install(loop)
 
 # Twisted and canonicaljson will fail to import when this file is executed to
 # get the __version__ during a fresh install. That's OK and subsequent calls to