diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index f184727cedb5..6561b490bcf5 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -541,8 +541,11 @@ jobs:
 
       - run: |
           set -o pipefail
-          POSTGRES=${{ (matrix.database == 'Postgres') && 1 || '' }} WORKERS=${{ (matrix.arrangement == 'workers') && 1 || '' }} COMPLEMENT_DIR=`pwd`/complement synapse/scripts-dev/complement.sh -json 2>&1 | synapse/.ci/scripts/gotestfmt
+          COMPLEMENT_DIR=`pwd`/complement synapse/scripts-dev/complement.sh -json 2>&1 | synapse/.ci/scripts/gotestfmt
         shell: bash
+        env:
+          POSTGRES: ${{ (matrix.database == 'Postgres') && 1 || '' }}
+          WORKERS: ${{ (matrix.arrangement == 'workers') && 1 || '' }}
         name: Run Complement Tests
 
   cargo-test:
diff --git a/changelog.d/14858.misc b/changelog.d/14858.misc
new file mode 100644
index 000000000000..c48f40cd3876
--- /dev/null
+++ b/changelog.d/14858.misc
@@ -0,0 +1 @@
+Run the integration test suites with the asyncio reactor enabled in CI.
diff --git a/docker/complement/conf/start_for_complement.sh b/docker/complement/conf/start_for_complement.sh
index 49d79745b064..af13209c54e9 100755
--- a/docker/complement/conf/start_for_complement.sh
+++ b/docker/complement/conf/start_for_complement.sh
@@ -6,7 +6,7 @@ set -e
 
 echo "Complement Synapse launcher"
 echo "  Args: $@"
-echo "  Env: SYNAPSE_COMPLEMENT_DATABASE=$SYNAPSE_COMPLEMENT_DATABASE SYNAPSE_COMPLEMENT_USE_WORKERS=$SYNAPSE_COMPLEMENT_USE_WORKERS"
+echo "  Env: SYNAPSE_COMPLEMENT_DATABASE=$SYNAPSE_COMPLEMENT_DATABASE SYNAPSE_COMPLEMENT_USE_WORKERS=$SYNAPSE_COMPLEMENT_USE_WORKERS SYNAPSE_COMPLEMENT_USE_ASYNCIO_REACTOR=$SYNAPSE_COMPLEMENT_USE_ASYNCIO_REACTOR"
 
 function log {
     d=$(date +"%Y-%m-%d %H:%M:%S,%3N")
@@ -76,6 +76,17 @@ else
 fi
 
 
+if [[ -n "$SYNAPSE_COMPLEMENT_USE_ASYNCIO_REACTOR" ]]; then
+  if [[ -n "$SYNAPSE_USE_EXPERIMENTAL_FORKING_LAUNCHER" ]]; then
+    export SYNAPSE_COMPLEMENT_FORKING_LAUNCHER_ASYNC_IO_REACTOR="1"
+  else
+    export SYNAPSE_ASYNC_IO_REACTOR="1"
+  fi
+else
+  export SYNAPSE_ASYNC_IO_REACTOR="0"
+fi
+
+
 # Add Complement's appservice registration directory, if there is one
 # (It can be absent when there are no application services in this test!)
 if [ -d /complement/appservice ]; then
diff --git a/docs/development/contributing_guide.md b/docs/development/contributing_guide.md
index 3cbfe9698762..36bc88468468 100644
--- a/docs/development/contributing_guide.md
+++ b/docs/development/contributing_guide.md
@@ -332,6 +332,7 @@ The above will run a monolithic (single-process) Synapse with SQLite as the data
     [here](https://github.com/matrix-org/synapse/blob/develop/docker/configure_workers_and_start.py#L54).
     A safe example would be `WORKER_TYPES="federation_inbound, federation_sender, synchrotron"`.
     See the [worker documentation](../workers.md) for additional information on workers.
+- Passing `ASYNCIO_REACTOR=1` as an environment variable to use the Twisted asyncio reactor instead of the default one.
 
 To increase the log level for the tests, set `SYNAPSE_TEST_LOG_LEVEL`, e.g:
 ```sh
diff --git a/scripts-dev/complement.sh b/scripts-dev/complement.sh
index e72d96fd165c..66aaa3d8480d 100755
--- a/scripts-dev/complement.sh
+++ b/scripts-dev/complement.sh
@@ -228,6 +228,11 @@ else
   test_tags="$test_tags,msc2716"
 fi
 
+if [[ -n "$ASYNCIO_REACTOR" ]]; then
+  # Enable the Twisted asyncio reactor
+  export PASS_SYNAPSE_COMPLEMENT_USE_ASYNCIO_REACTOR=true
+fi
+
 
 if [[ -n "$SYNAPSE_TEST_LOG_LEVEL" ]]; then
   # Set the log level to what is desired
diff --git a/synapse/app/complement_fork_starter.py b/synapse/app/complement_fork_starter.py
index 8c0f4a57e70a..920538f44df2 100644
--- a/synapse/app/complement_fork_starter.py
+++ b/synapse/app/complement_fork_starter.py
@@ -110,6 +110,8 @@ def _worker_entrypoint(
     and then kick off the worker's main() function.
     """
 
+    from synapse.util.stringutils import strtobool
+
     sys.argv = args
 
     # reset the custom signal handlers that we installed, so that the children start
@@ -117,9 +119,24 @@ def _worker_entrypoint(
     for sig, handler in _original_signal_handlers.items():
         signal.signal(sig, handler)
 
-    from twisted.internet.epollreactor import EPollReactor
+    # Install the asyncio reactor if the
+    # SYNAPSE_COMPLEMENT_FORKING_LAUNCHER_ASYNC_IO_REACTOR is set to 1. The
+    # SYNAPSE_ASYNC_IO_REACTOR variable would be used, but then causes
+    # synapse/__init__.py to also try to install an asyncio reactor.
+    if strtobool(
+        os.environ.get("SYNAPSE_COMPLEMENT_FORKING_LAUNCHER_ASYNC_IO_REACTOR", "0")
+    ):
+        import asyncio
+
+        from twisted.internet.asyncioreactor import AsyncioSelectorReactor
+
+        reactor = AsyncioSelectorReactor(asyncio.get_event_loop())
+        proxy_reactor._install_real_reactor(reactor)
+    else:
+        from twisted.internet.epollreactor import EPollReactor
+
+        proxy_reactor._install_real_reactor(EPollReactor())
 
-    proxy_reactor._install_real_reactor(EPollReactor())
     func()