diff --git a/changelog.d/14875.docker b/changelog.d/14875.docker
new file mode 100644
index 0000000000..584fc10470
--- /dev/null
+++ b/changelog.d/14875.docker
@@ -0,0 +1 @@
+Bump default Python version in the Dockerfile from 3.9 to 3.11.
diff --git a/docker/Dockerfile b/docker/Dockerfile
index b2ec005917..a85fd3d691 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -20,7 +20,7 @@
 # `poetry export | pip install -r /dev/stdin`, but beware: we have experienced bugs in
 # in `poetry export` in the past.
 
-ARG PYTHON_VERSION=3.9
+ARG PYTHON_VERSION=3.11
 
 ###
 ### Stage 0: generate requirements.txt
@@ -34,11 +34,11 @@ FROM docker.io/python:${PYTHON_VERSION}-slim-bullseye as requirements
 # Here we use it to set up a cache for apt (and below for pip), to improve
 # rebuild speeds on slow connections.
 RUN \
-   --mount=type=cache,target=/var/cache/apt,sharing=locked \
-   --mount=type=cache,target=/var/lib/apt,sharing=locked \
-    apt-get update -qq && apt-get install -yqq \
-      build-essential git libffi-dev libssl-dev \
-    && rm -rf /var/lib/apt/lists/*
+  --mount=type=cache,target=/var/cache/apt,sharing=locked \
+  --mount=type=cache,target=/var/lib/apt,sharing=locked \
+  apt-get update -qq && apt-get install -yqq \
+  build-essential git libffi-dev libssl-dev \
+  && rm -rf /var/lib/apt/lists/*
 
 # We install poetry in its own build stage to avoid its dependencies conflicting with
 # synapse's dependencies.
@@ -64,9 +64,9 @@ ARG TEST_ONLY_IGNORE_POETRY_LOCKFILE
 # Otherwise, just create an empty requirements file so that the Dockerfile can
 # proceed.
 RUN if [ -z "$TEST_ONLY_IGNORE_POETRY_LOCKFILE" ]; then \
-    /root/.local/bin/poetry export --extras all -o /synapse/requirements.txt ${TEST_ONLY_SKIP_DEP_HASH_VERIFICATION:+--without-hashes}; \
+  /root/.local/bin/poetry export --extras all -o /synapse/requirements.txt ${TEST_ONLY_SKIP_DEP_HASH_VERIFICATION:+--without-hashes}; \
   else \
-    touch /synapse/requirements.txt; \
+  touch /synapse/requirements.txt; \
   fi
 
 ###
@@ -76,24 +76,24 @@ FROM docker.io/python:${PYTHON_VERSION}-slim-bullseye as builder
 
 # install the OS build deps
 RUN \
-   --mount=type=cache,target=/var/cache/apt,sharing=locked \
-   --mount=type=cache,target=/var/lib/apt,sharing=locked \
- apt-get update -qq && apt-get install -yqq \
-    build-essential \
-    libffi-dev \
-    libjpeg-dev \
-    libpq-dev \
-    libssl-dev \
-    libwebp-dev \
-    libxml++2.6-dev \
-    libxslt1-dev \
-    openssl \
-    zlib1g-dev \
-    git \
-    curl \
-    libicu-dev \
-    pkg-config \
-    && rm -rf /var/lib/apt/lists/*
+  --mount=type=cache,target=/var/cache/apt,sharing=locked \
+  --mount=type=cache,target=/var/lib/apt,sharing=locked \
+  apt-get update -qq && apt-get install -yqq \
+  build-essential \
+  libffi-dev \
+  libjpeg-dev \
+  libpq-dev \
+  libssl-dev \
+  libwebp-dev \
+  libxml++2.6-dev \
+  libxslt1-dev \
+  openssl \
+  zlib1g-dev \
+  git \
+  curl \
+  libicu-dev \
+  pkg-config \
+  && rm -rf /var/lib/apt/lists/*
 
 
 # Install rust and ensure its in the PATH
@@ -134,9 +134,9 @@ ARG TEST_ONLY_IGNORE_POETRY_LOCKFILE
 RUN --mount=type=cache,target=/synapse/target,sharing=locked \
   --mount=type=cache,target=${CARGO_HOME}/registry,sharing=locked \
   if [ -z "$TEST_ONLY_IGNORE_POETRY_LOCKFILE" ]; then \
-    pip install --prefix="/install" --no-deps --no-warn-script-location /synapse[all]; \
+  pip install --prefix="/install" --no-deps --no-warn-script-location /synapse[all]; \
   else \
-    pip install --prefix="/install" --no-warn-script-location /synapse[all]; \
+  pip install --prefix="/install" --no-warn-script-location /synapse[all]; \
   fi
 
 ###
@@ -151,20 +151,20 @@ LABEL org.opencontainers.image.source='https://github.com/matrix-org/synapse.git
 LABEL org.opencontainers.image.licenses='Apache-2.0'
 
 RUN \
-   --mount=type=cache,target=/var/cache/apt,sharing=locked \
-   --mount=type=cache,target=/var/lib/apt,sharing=locked \
+  --mount=type=cache,target=/var/cache/apt,sharing=locked \
+  --mount=type=cache,target=/var/lib/apt,sharing=locked \
   apt-get update -qq && apt-get install -yqq \
-    curl \
-    gosu \
-    libjpeg62-turbo \
-    libpq5 \
-    libwebp6 \
-    xmlsec1 \
-    libjemalloc2 \
-    libicu67 \
-    libssl-dev \
-    openssl \
-    && rm -rf /var/lib/apt/lists/*
+  curl \
+  gosu \
+  libjpeg62-turbo \
+  libpq5 \
+  libwebp6 \
+  xmlsec1 \
+  libjemalloc2 \
+  libicu67 \
+  libssl-dev \
+  openssl \
+  && rm -rf /var/lib/apt/lists/*
 
 COPY --from=builder /install /usr/local
 COPY ./docker/start.py /start.py
@@ -175,4 +175,4 @@ EXPOSE 8008/tcp 8009/tcp 8448/tcp
 ENTRYPOINT ["/start.py"]
 
 HEALTHCHECK --start-period=5s --interval=15s --timeout=5s \
-    CMD curl -fSs http://localhost:8008/health || exit 1
+  CMD curl -fSs http://localhost:8008/health || exit 1