diff --git a/tests/roots/test-util-copyasset_overwrite/myext.py b/tests/roots/test-util-copyasset_overwrite/myext.py
index 3f961fb8758..6c2ad23395b 100644
--- a/tests/roots/test-util-copyasset_overwrite/myext.py
+++ b/tests/roots/test-util-copyasset_overwrite/myext.py
@@ -6,14 +6,14 @@
 def _copy_asset_overwrite_hook(app):
     css = app.outdir / '_static' / 'custom-styles.css'
     # html_static_path is copied by default
-    assert css.read_text() == '/* html_static_path */\n', 'invalid default text'
+    assert css.read_text(encoding='utf-8') == '/* html_static_path */\n', 'invalid default text'
     # warning generated by here
     copy_asset(
         Path(__file__).parent.joinpath('myext_static', 'custom-styles.css'),
         app.outdir / '_static',
     )
     # This demonstrates that no overwriting occurs
-    assert css.read_text() == '/* html_static_path */\n', 'file overwritten!'
+    assert css.read_text(encoding='utf-8') == '/* html_static_path */\n', 'file overwritten!'
     return []
 
 
diff --git a/tests/test_builders/test_build_linkcheck.py b/tests/test_builders/test_build_linkcheck.py
index cd36981f74f..8a132ec0656 100644
--- a/tests/test_builders/test_build_linkcheck.py
+++ b/tests/test_builders/test_build_linkcheck.py
@@ -240,6 +240,7 @@ def test_raw_node(app: SphinxTestApp) -> None:
         index.write_text(
             ".. raw:: 'html'\n"
             "   :url: http://{address}/".format(address=address),
+            encoding='utf-8',
         )
         app.build()
 
diff --git a/tests/test_extensions/test_ext_autosummary.py b/tests/test_extensions/test_ext_autosummary.py
index bdce719e72e..2f8bb946713 100644
--- a/tests/test_extensions/test_ext_autosummary.py
+++ b/tests/test_extensions/test_ext_autosummary.py
@@ -696,7 +696,7 @@ def test_autogen(rootdir, tmp_path):
 
 def test_autogen_remove_old(rootdir, tmp_path):
     """Test the ``--remove-old`` option."""
-    tmp_path.joinpath('other.rst').write_text('old content')
+    tmp_path.joinpath('other.rst').write_text('old content', encoding='utf-8')
     with chdir(rootdir / 'test-templating'):
         args = ['-o', str(tmp_path), '-t', '.', 'autosummary_templating.txt']
         autogen_main(args)