diff --git a/CHANGES.md b/CHANGES.md
index d7928a30c5c..00cf0e6ec30 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -69,6 +69,7 @@
 
 <!-- For example, Docker, GitHub Actions, pre-commit, editors -->
 
+- Black is now tested with [`PYTHONWARNDEFAULTENCODING = 1`](https://docs.python.org/3/library/io.html#io-encoding-warning) (#3658)
 - Update GitHub Action to display black output in the job summary (#3688)
 - Deprecated `set-output` command in CI test to keep up to date with GitHub's
   deprecation announcement (#3757)
diff --git a/tests/test_black.py b/tests/test_black.py
index abb304a246d..66d580951bb 100644
--- a/tests/test_black.py
+++ b/tests/test_black.py
@@ -149,8 +149,7 @@ def test_empty_ff(self) -> None:
         tmp_file = Path(black.dump_to_file())
         try:
             self.assertFalse(ff(tmp_file, write_back=black.WriteBack.YES))
-            with open(tmp_file, encoding="utf8") as f:
-                actual = f.read()
+            actual = tmp_file.read_text(encoding="utf8")
         finally:
             os.unlink(tmp_file)
         self.assertFormatEqual(expected, actual)
@@ -286,8 +285,7 @@ def test_expression_ff(self) -> None:
         tmp_file = Path(black.dump_to_file(source))
         try:
             self.assertTrue(ff(tmp_file, write_back=black.WriteBack.YES))
-            with open(tmp_file, encoding="utf8") as f:
-                actual = f.read()
+            actual = tmp_file.read_text(encoding="utf8")
         finally:
             os.unlink(tmp_file)
         self.assertFormatEqual(expected, actual)
@@ -390,8 +388,7 @@ def test_skip_source_first_line(self) -> None:
             black.main, [str(tmp_file), "-x", f"--config={EMPTY_CONFIG}"]
         )
         self.assertEqual(result.exit_code, 0)
-        with open(tmp_file, encoding="utf8") as f:
-            actual = f.read()
+        actual = tmp_file.read_text(encoding="utf8")
         self.assertFormatEqual(source, actual)
 
     def test_skip_source_first_line_when_mixing_newlines(self) -> None:
@@ -1081,7 +1078,7 @@ def test_works_in_mono_process_only_environment(self) -> None:
                 (workspace / "one.py").resolve(),
                 (workspace / "two.py").resolve(),
             ]:
-                f.write_text('print("hello")\n')
+                f.write_text('print("hello")\n', encoding="utf-8")
             self.invokeBlack([str(workspace)])
 
     @event_loop()
@@ -1118,11 +1115,9 @@ def test_single_file_force_pyi(self) -> None:
         contents, expected = read_data("miscellaneous", "force_pyi")
         with cache_dir() as workspace:
             path = (workspace / "file.py").resolve()
-            with open(path, "w") as fh:
-                fh.write(contents)
+            path.write_text(contents, encoding="utf-8")
             self.invokeBlack([str(path), "--pyi"])
-            with open(path, "r") as fh:
-                actual = fh.read()
+            actual = path.read_text(encoding="utf8")
             # verify cache with --pyi is separate
             pyi_cache = black.read_cache(pyi_mode)
             self.assertIn(str(path), pyi_cache)
@@ -1143,12 +1138,10 @@ def test_multi_file_force_pyi(self) -> None:
                 (workspace / "file2.py").resolve(),
             ]
             for path in paths:
-                with open(path, "w") as fh:
-                    fh.write(contents)
+                path.write_text(contents, encoding="utf-8")
             self.invokeBlack([str(p) for p in paths] + ["--pyi"])
             for path in paths:
-                with open(path, "r") as fh:
-                    actual = fh.read()
+                actual = path.read_text(encoding="utf8")
                 self.assertEqual(actual, expected)
             # verify cache with --pyi is separate
             pyi_cache = black.read_cache(pyi_mode)
@@ -1172,11 +1165,9 @@ def test_single_file_force_py36(self) -> None:
         source, expected = read_data("miscellaneous", "force_py36")
         with cache_dir() as workspace:
             path = (workspace / "file.py").resolve()
-            with open(path, "w") as fh:
-                fh.write(source)
+            path.write_text(source, encoding="utf-8")
             self.invokeBlack([str(path), *PY36_ARGS])
-            with open(path, "r") as fh:
-                actual = fh.read()
+            actual = path.read_text(encoding="utf8")
             # verify cache with --target-version is separate
             py36_cache = black.read_cache(py36_mode)
             self.assertIn(str(path), py36_cache)
@@ -1195,12 +1186,10 @@ def test_multi_file_force_py36(self) -> None:
                 (workspace / "file2.py").resolve(),
             ]
             for path in paths:
-                with open(path, "w") as fh:
-                    fh.write(source)
+                path.write_text(source, encoding="utf-8")
             self.invokeBlack([str(p) for p in paths] + PY36_ARGS)
             for path in paths:
-                with open(path, "r") as fh:
-                    actual = fh.read()
+                actual = path.read_text(encoding="utf8")
                 self.assertEqual(actual, expected)
             # verify cache with --target-version is separate
             pyi_cache = black.read_cache(py36_mode)
@@ -1631,8 +1620,8 @@ def test_read_pyproject_toml_from_stdin(self) -> None:
             src_pyproject = src_dir / "pyproject.toml"
             src_pyproject.touch()
 
-            test_toml_file = THIS_DIR / "test.toml"
-            src_pyproject.write_text(test_toml_file.read_text())
+            test_toml_content = (THIS_DIR / "test.toml").read_text(encoding="utf-8")
+            src_pyproject.write_text(test_toml_content, encoding="utf-8")
 
             src_python = src_dir / "foo.py"
             src_python.touch()
@@ -1985,10 +1974,10 @@ def test_cache_broken_file(self) -> None:
         mode = DEFAULT_MODE
         with cache_dir() as workspace:
             cache_file = get_cache_file(mode)
-            cache_file.write_text("this is not a pickle")
+            cache_file.write_text("this is not a pickle", encoding="utf-8")
             assert black.read_cache(mode) == {}
             src = (workspace / "test.py").resolve()
-            src.write_text("print('hello')")
+            src.write_text("print('hello')", encoding="utf-8")
             invokeBlack([str(src)])
             cache = black.read_cache(mode)
             assert str(src) in cache
@@ -1997,10 +1986,10 @@ def test_cache_single_file_already_cached(self) -> None:
         mode = DEFAULT_MODE
         with cache_dir() as workspace:
             src = (workspace / "test.py").resolve()
-            src.write_text("print('hello')")
+            src.write_text("print('hello')", encoding="utf-8")
             black.write_cache({}, [src], mode)
             invokeBlack([str(src)])
-            assert src.read_text() == "print('hello')"
+            assert src.read_text(encoding="utf-8") == "print('hello')"
 
     @event_loop()
     def test_cache_multiple_files(self) -> None:
@@ -2009,17 +1998,13 @@ def test_cache_multiple_files(self) -> None:
             "concurrent.futures.ProcessPoolExecutor", new=ThreadPoolExecutor
         ):
             one = (workspace / "one.py").resolve()
-            with one.open("w") as fobj:
-                fobj.write("print('hello')")
+            one.write_text("print('hello')", encoding="utf-8")
             two = (workspace / "two.py").resolve()
-            with two.open("w") as fobj:
-                fobj.write("print('hello')")
+            two.write_text("print('hello')", encoding="utf-8")
             black.write_cache({}, [one], mode)
             invokeBlack([str(workspace)])
-            with one.open("r") as fobj:
-                assert fobj.read() == "print('hello')"
-            with two.open("r") as fobj:
-                assert fobj.read() == 'print("hello")\n'
+            assert one.read_text(encoding="utf-8") == "print('hello')"
+            assert two.read_text(encoding="utf-8") == "print('hello')\n"
             cache = black.read_cache(mode)
             assert str(one) in cache
             assert str(two) in cache
@@ -2029,8 +2014,7 @@ def test_no_cache_when_writeback_diff(self, color: bool) -> None:
         mode = DEFAULT_MODE
         with cache_dir() as workspace:
             src = (workspace / "test.py").resolve()
-            with src.open("w") as fobj:
-                fobj.write("print('hello')")
+            src.write_text("print('hello')", encoding="utf-8")
             with patch("black.read_cache") as read_cache, patch(
                 "black.write_cache"
             ) as write_cache:
@@ -2049,8 +2033,7 @@ def test_output_locking_when_writeback_diff(self, color: bool) -> None:
         with cache_dir() as workspace:
             for tag in range(0, 4):
                 src = (workspace / f"test{tag}.py").resolve()
-                with src.open("w") as fobj:
-                    fobj.write("print('hello')")
+                src.write_text("print('hello')", encoding="utf-8")
             with patch(
                 "black.concurrency.Manager", wraps=multiprocessing.Manager
             ) as mgr:
@@ -2120,11 +2103,9 @@ def test_failed_formatting_does_not_get_cached(self) -> None:
             "concurrent.futures.ProcessPoolExecutor", new=ThreadPoolExecutor
         ):
             failing = (workspace / "failing.py").resolve()
-            with failing.open("w") as fobj:
-                fobj.write("not actually python")
+            failing.write_text("not actually python", encoding="utf-8")
             clean = (workspace / "clean.py").resolve()
-            with clean.open("w") as fobj:
-                fobj.write('print("hello")\n')
+            clean.write_text('print("hello")\n', encoding="utf-8")
             invokeBlack([str(workspace)], exit_code=123)
             cache = black.read_cache(mode)
             assert str(failing) not in cache
diff --git a/tox.ini b/tox.ini
index 4934514264b..f8e1a785331 100644
--- a/tox.ini
+++ b/tox.ini
@@ -3,7 +3,9 @@ isolated_build = true
 envlist = {,ci-}py{37,38,39,310,311,py3},fuzz,run_self
 
 [testenv]
-setenv = PYTHONPATH = {toxinidir}/src
+setenv =
+    PYTHONPATH = {toxinidir}/src
+    PYTHONWARNDEFAULTENCODING = 1
 skip_install = True
 # We use `recreate=True` because otherwise, on the second run of `tox -e py`,
 # the `no_jupyter` tests would run with the jupyter extra dependencies installed.