diff --git a/.github/workflows/checks.yml b/.github/workflows/checks.yml
index 2748bf5fe..02c647da9 100644
--- a/.github/workflows/checks.yml
+++ b/.github/workflows/checks.yml
@@ -7,19 +7,21 @@ jobs:
     strategy:
       matrix:
         include:
-        - python-version: 3.8
+        - python-version: 3.9
           env:
             TOXENV: security
-        - python-version: 3.8
+        - python-version: 3.9
           env:
             TOXENV: flake8
+        # Pylint requires installing reppy, which does not support Python 3.9
+        # https://github.com/seomoz/reppy/issues/122
         - python-version: 3.8
           env:
             TOXENV: pylint
-        - python-version: 3.8
+        - python-version: 3.9
           env:
             TOXENV: typing
-        - python-version: 3.7  # Keep in sync with .readthedocs.yml
+        - python-version: 3.8  # Keep in sync with .readthedocs.yml
           env:
             TOXENV: docs
 
diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index aec6b8696..b48066ea4 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -9,10 +9,10 @@ jobs:
     steps:
     - uses: actions/checkout@v2
 
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v2
       with:
-        python-version: 3.8
+        python-version: 3.9
 
     - name: Check Tag
       id: check-release-tag
diff --git a/.github/workflows/tests-macos.yml b/.github/workflows/tests-macos.yml
index 51d27c405..4f8f7a19d 100644
--- a/.github/workflows/tests-macos.yml
+++ b/.github/workflows/tests-macos.yml
@@ -6,7 +6,7 @@ jobs:
     runs-on: macos-10.15
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.6, 3.7, 3.8, 3.9]
 
     steps:
     - uses: actions/checkout@v2
diff --git a/.github/workflows/tests-ubuntu.yml b/.github/workflows/tests-ubuntu.yml
index 89c0334e2..df5ee9d69 100644
--- a/.github/workflows/tests-ubuntu.yml
+++ b/.github/workflows/tests-ubuntu.yml
@@ -13,6 +13,9 @@ jobs:
         - python-version: 3.8
           env:
             TOXENV: py
+        - python-version: 3.9
+          env:
+            TOXENV: py
         - python-version: pypy3
           env:
             TOXENV: pypy3
@@ -31,10 +34,12 @@ jobs:
             PYPY_VERSION: 3.6-v7.2.0
 
         # extras
+        # extra-deps includes reppy, which does not support Python 3.9
+        # https://github.com/seomoz/reppy/issues/122
         - python-version: 3.8
           env:
             TOXENV: extra-deps
-        - python-version: 3.8
+        - python-version: 3.9
           env:
             TOXENV: asyncio
 
diff --git a/.github/workflows/tests-windows.yml b/.github/workflows/tests-windows.yml
index ed2e4075d..5459a845b 100644
--- a/.github/workflows/tests-windows.yml
+++ b/.github/workflows/tests-windows.yml
@@ -16,6 +16,10 @@ jobs:
         - python-version: 3.8
           env:
             TOXENV: py
+        # https://twistedmatrix.com/trac/ticket/9990
+        #- python-version: 3.9
+          #env:
+            #TOXENV: py
 
     steps:
     - uses: actions/checkout@v2
diff --git a/.readthedocs.yml b/.readthedocs.yml
index e4d3f02cc..80a1cd036 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -3,10 +3,14 @@ formats: all
 sphinx:
   configuration: docs/conf.py
   fail_on_warning: true
+
+build:
+  image: latest
+
 python:
   # For available versions, see:
   # https://docs.readthedocs.io/en/stable/config-file/v2.html#build-image
-  version: 3.7  # Keep in sync with .travis.yml
+  version: 3.8  # Keep in sync with .github/workflows/checks.yml
   install:
     - requirements: docs/requirements.txt
     - path: .
diff --git a/setup.py b/setup.py
index b5c42a3c2..cf9261271 100644
--- a/setup.py
+++ b/setup.py
@@ -85,6 +85,7 @@ setup(
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
         'Programming Language :: Python :: Implementation :: CPython',
         'Programming Language :: Python :: Implementation :: PyPy',
         'Topic :: Internet :: WWW/HTTP',
diff --git a/tox.ini b/tox.ini
index 69f52bd9f..9815f80f7 100644
--- a/tox.ini
+++ b/tox.ini
@@ -13,7 +13,9 @@ deps =
     -rtests/requirements-py3.txt
     # mitmproxy does not support PyPy
     # mitmproxy does not support Windows when running Python < 3.7
-    mitmproxy >= 4.0.4; python_version >= '3.7' and implementation_name != 'pypy'
+    # Python 3.9+ requires https://github.com/mitmproxy/mitmproxy/commit/8e5e43de24c9bc93092b63efc67fbec029a9e7fe
+    mitmproxy >= 5.3.0; python_version >= '3.9' and implementation_name != 'pypy'
+    mitmproxy >= 4.0.4; python_version >= '3.7' and python_version < '3.9' and implementation_name != 'pypy'
     mitmproxy >= 4.0.4, < 5; python_version >= '3.6' and python_version < '3.7' and platform_system != 'Windows' and implementation_name != 'pypy'
     # Extras
     botocore>=1.4.87