diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 0589f7e0..88e0dc1d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -38,7 +38,7 @@ jobs:
     with:
       submodules: false
       # Any env name which does not start with `pyXY` will use this Python version.
-      default_python: '3.9'
+      default_python: '3.11'
       envs: |
         - macos: py39
         - windows: py39
@@ -66,10 +66,10 @@ jobs:
           fetch-depth: 0
           repository: asdf-format/asdf
           path: asdf
-      - name: Set up Python 3.9
+      - name: Set up Python 3.11
         uses: actions/setup-python@v5
         with:
-          python-version: 3.9
+          python-version: 3.11
       - name: Install asdf-standard
         run: cd asdf-standard && pip install .
       - name: Install asdf
diff --git a/.github/workflows/downstream.yml b/.github/workflows/downstream.yml
index 78a78a38..533e5c73 100644
--- a/.github/workflows/downstream.yml
+++ b/.github/workflows/downstream.yml
@@ -97,10 +97,10 @@ jobs:
           fetch-depth: 0
           repository: ${{ matrix.repository }}
           path: target
-      - name: Set up Python 3.9
+      - name: Set up Python 3.11
         uses: actions/setup-python@v5
         with:
-          python-version: 3.9
+          python-version: 3.11
       - name: Install asdf-standard
         run: cd asdf-standard && pip install .
       - name: Install remaining ${{ matrix.package_name }} dependencies
diff --git a/tox.ini b/tox.ini
index 40f33c3e..fb1dfb7f 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = py39, py310, py311
+envlist = py39, py310, py311, py312
 isolated_build = True
 
 [testenv]