diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
index e57d864..be340e4 100644
--- a/.github/copilot-instructions.md
+++ b/.github/copilot-instructions.md
@@ -6,7 +6,7 @@
 
 ### Technology Stack
 
-- **Python**: 3.9+ (`from __future__ import annotations`)
+- **Python**: 3.10+ (`from __future__ import annotations`)
 - **Core**: NumPy (≥1.22), h5py (≥3.6), QtPy (≥1.9)
 - **GUI**: Qt via QtPy (PyQt5/PyQt6/PySide6)
 - **Testing**: pytest
diff --git a/.github/workflows/test_pyqt5.yml b/.github/workflows/test_pyqt5.yml
index 63d41c1..c80132e 100644
--- a/.github/workflows/test_pyqt5.yml
+++ b/.github/workflows/test_pyqt5.yml
@@ -22,7 +22,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.9", "3.11", "3.14"]
+        python-version: ["3.10", "3.11", "3.14"]
 
     steps:
     - uses: actions/checkout@v7
diff --git a/.github/workflows/test_pyqt6.yml b/.github/workflows/test_pyqt6.yml
index a556349..bf7cebf 100644
--- a/.github/workflows/test_pyqt6.yml
+++ b/.github/workflows/test_pyqt6.yml
@@ -22,7 +22,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.9", "3.11", "3.14"]
+        python-version: ["3.10", "3.11", "3.14"]
 
     steps:
     - uses: actions/checkout@v7
diff --git a/.github/workflows/test_pyside6.yml b/.github/workflows/test_pyside6.yml
index e2a2085..0872748 100644
--- a/.github/workflows/test_pyside6.yml
+++ b/.github/workflows/test_pyside6.yml
@@ -22,7 +22,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.9", "3.11", "3.14"]
+        python-version: ["3.10", "3.11", "3.14"]
 
     steps:
     - uses: actions/checkout@v7
@@ -36,7 +36,8 @@ jobs:
         /sbin/start-stop-daemon --start --quiet --pidfile /tmp/custom_xvfb_99.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :99 -screen 0 1920x1200x24 -ac +extension GLX
         python -m pip install --upgrade pip
         python -m pip install ruff pytest
-        pip install PySide6
+        # PySide6 6.12.0 drops a reference to None on each void call: fatal before Python 3.12
+        pip install "PySide6!=6.12.0; python_version < '3.12'" "PySide6; python_version >= '3.12'"
         pip install .
     - name: Lint with Ruff
       run: ruff check --output-format=github guidata
diff --git a/doc/release_notes/release_3.16.md b/doc/release_notes/release_3.16.md
index 86dac3e..80da0de 100644
--- a/doc/release_notes/release_3.16.md
+++ b/doc/release_notes/release_3.16.md
@@ -1,5 +1,9 @@
 # Version 3.16 #
 
+## Unreleased ##
+
+* Python 3.10 or later is now required. Python 3.9 users must keep an earlier compatible release.
+
 ## guidata Version 3.16.0 ##
 
 ✨ New features:
diff --git a/doc/requirements.rst b/doc/requirements.rst
index c555a69..495e080 100644
--- a/doc/requirements.rst
+++ b/doc/requirements.rst
@@ -8,7 +8,7 @@ The `guidata` package requires the following Python modules:
       - Version
       - Summary
     * - Python
-      - >=3.9, <4
+      - >=3.10, <4
       - Python programming language
     * - h5py
       - >= 3.6
diff --git a/guidata/widgets/arrayeditor/arrayhandler.py b/guidata/widgets/arrayeditor/arrayhandler.py
index 70af1ba..5af771c 100644
--- a/guidata/widgets/arrayeditor/arrayhandler.py
+++ b/guidata/widgets/arrayeditor/arrayhandler.py
@@ -21,15 +21,11 @@
 from __future__ import annotations
 
 import copy
-from typing import Any, Generic, TypeVar, Union, cast
+from typing import Any, Generic, TypeVar, cast
 
 import numpy as np
 
-# TODO: (When Python 3.8-3.9 support is dropped) Use `np.ndarray | np.ma.MaskedArray`
-# instead of `Union[np.ndarray, np.ma.MaskedArray]`
-AnySupportedArray = TypeVar(
-    "AnySupportedArray", bound=Union[np.ndarray, np.ma.MaskedArray]
-)
+AnySupportedArray = TypeVar("AnySupportedArray", bound=np.ndarray | np.ma.MaskedArray)
 AnyArrayHandler = TypeVar("AnyArrayHandler", bound="BaseArrayHandler")
 
 
diff --git a/pyproject.toml b/pyproject.toml
index 0b593a7..ef12592 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -24,7 +24,6 @@ classifiers = [
     "Operating System :: Microsoft :: Windows :: Windows 10",
     "Operating System :: Microsoft :: Windows :: Windows 11",
     "Operating System :: POSIX :: Linux",
-    "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",
     "Programming Language :: Python :: 3.11",
     "Programming Language :: Python :: 3.12",
@@ -37,7 +36,7 @@ classifiers = [
     "Topic :: Software Development :: Widget Sets",
     "Topic :: Utilities",
 ]
-requires-python = ">=3.9, <4"
+requires-python = ">=3.10, <4"
 dependencies = ["h5py >= 3.6", "NumPy >= 1.22", "QtPy >= 1.9", "requests", "tomli; python_version < '3.11'"]
 dynamic = ["version"]
 
@@ -88,7 +87,7 @@ addopts = "guidata --import-mode=importlib"
 exclude = [".git", ".vscode", "build", "dist", "guidata/external","venv*",".venv*"]
 line-length = 88                                                   # Same as Black.
 indent-width = 4                                                   # Same as Black.
-target-version = "py39"                                            # Assume Python 3.9.
+target-version = "py310"                                           # Assume Python 3.10.
 
 [tool.ruff.lint]
 # all rules can be found here: https://beta.ruff.rs/docs/rules/