diff --git a/.ci/patch_for_twisted_trunk.sh b/.ci/patch_for_twisted_trunk.sh
deleted file mode 100755
index f524581986a9..000000000000
--- a/.ci/patch_for_twisted_trunk.sh
+++ /dev/null
@@ -1,8 +0,0 @@
-#!/bin/sh
-
-# replaces the dependency on Twisted in `python_dependencies` with trunk.
-
-set -e
-cd "$(dirname "$0")"/..
-
-sed -i -e 's#"Twisted.*"#"Twisted @ git+https://github.com/twisted/twisted"#' synapse/python_dependencies.py
diff --git a/.github/workflows/twisted_trunk.yml b/.github/workflows/twisted_trunk.yml
index fb9d46b7bfdd..8fc1affb7746 100644
--- a/.github/workflows/twisted_trunk.yml
+++ b/.github/workflows/twisted_trunk.yml
@@ -6,16 +6,25 @@ on:
 
   workflow_dispatch:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   mypy:
     runs-on: ubuntu-latest
 
     steps:
       - uses: actions/checkout@v2
-      - uses: actions/setup-python@v2
-      - run: .ci/patch_for_twisted_trunk.sh
-      - run: pip install tox
-      - run: tox -e mypy
+      - uses: matrix-org/setup-python-poetry@v1
+        with:
+          python-version: "3.x"
+          extras: "all"
+      - run: |
+          poetry remove twisted
+          poetry add --extras tls git+https://github.com/twisted/twisted.git#trunk
+          poetry install --no-interaction --extras "all test"
+      - run: poetry run mypy
 
   trial:
     runs-on: ubuntu-latest
@@ -23,14 +32,15 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - run: sudo apt-get -qq install xmlsec1
-      - uses: actions/setup-python@v2
+      - uses: matrix-org/setup-python-poetry@v1
         with:
-          python-version: 3.7
-      - run: .ci/patch_for_twisted_trunk.sh
-      - run: pip install tox
-      - run: tox -e py
-        env:
-          TRIAL_FLAGS: "--jobs=2"
+          python-version: "3.x"
+          extras: "all test"
+      - run: |
+          poetry remove twisted
+          poetry add --extras tls git+https://github.com/twisted/twisted.git#trunk
+          poetry install --no-interaction --extras "all test"
+      - run: poetry run trial --jobs 2 tests
 
       - name: Dump logs
         # Logs are most useful when the command fails, always include them.
@@ -55,11 +65,23 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: Patch dependencies
-        run: .ci/patch_for_twisted_trunk.sh
+        # Note: The poetry commands want to create a virtualenv in /src/.venv/,
+        #       but the sytest-synapse container expects it to be in /venv/.
+        #       We symlink it before running poetry so that poetry actually
+        #       ends up installing to `/venv`.
+        run: |
+          ln -s -T /venv /src/.venv
+          poetry remove twisted
+          poetry add --extras tls git+https://github.com/twisted/twisted.git#trunk
+          poetry install --no-interaction --extras "all test"
         working-directory: /src
       - name: Run SyTest
         run: /bootstrap.sh synapse
         working-directory: /src
+        env:
+          # Use offline mode to avoid reinstalling the pinned version of
+          # twisted.
+          OFFLINE: 1
       - name: Summarise results.tap
         if: ${{ always() }}
         run: /sytest/scripts/tap_to_gha.pl /logs/results.tap
diff --git a/changelog.d/12425.misc b/changelog.d/12425.misc
new file mode 100644
index 000000000000..3b076be0bd52
--- /dev/null
+++ b/changelog.d/12425.misc
@@ -0,0 +1 @@
+Run twisted trunk CI job in the locked poetry environment.
diff --git a/poetry.lock b/poetry.lock
index 17430cbfa6c1..c7324f9d20da 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -717,7 +717,7 @@ test = ["appdirs (==1.4.4)", "pytest (>=6)", "pytest-cov (>=2.7)", "pytest-mock
 
 [[package]]
 name = "prometheus-client"
-version = "0.13.1"
+version = "0.14.0"
 description = "Python client for the Prometheus monitoring system."
 category = "main"
 optional = false
@@ -2225,8 +2225,8 @@ platformdirs = [
     {file = "platformdirs-2.5.1.tar.gz", hash = "sha256:7535e70dfa32e84d4b34996ea99c5e432fa29a708d0f4e394bbcb2a8faa4f16d"},
 ]
 prometheus-client = [
-    {file = "prometheus_client-0.13.1-py3-none-any.whl", hash = "sha256:357a447fd2359b0a1d2e9b311a0c5778c330cfbe186d880ad5a6b39884652316"},
-    {file = "prometheus_client-0.13.1.tar.gz", hash = "sha256:ada41b891b79fca5638bd5cfe149efa86512eaa55987893becd2c6d8d0a5dfc5"},
+    {file = "prometheus_client-0.14.0-py3-none-any.whl", hash = "sha256:f4aba3fdd1735852049f537c1f0ab177159b7ab76f271ecc4d2f45aa2a1d01f2"},
+    {file = "prometheus_client-0.14.0.tar.gz", hash = "sha256:8f7a922dd5455ad524b6ba212ce8eb2b4b05e073f4ec7218287f88b1cac34750"},
 ]
 psycopg2 = [
     {file = "psycopg2-2.9.3-cp310-cp310-win32.whl", hash = "sha256:083707a696e5e1c330af2508d8fab36f9700b26621ccbcb538abe22e15485362"},