diff --git a/changelog/7295.trivial.rst b/changelog/7295.trivial.rst
new file mode 100644
index 000000000..113a7ee60
--- /dev/null
+++ b/changelog/7295.trivial.rst
@@ -0,0 +1 @@
+``src/_pytest/config/__init__.py`` now uses the ``warnings`` module to report warnings instead of ``sys.stderr.write``.
diff --git a/src/_pytest/config/__init__.py b/src/_pytest/config/__init__.py
index bf5b78082..b5cff7301 100644
--- a/src/_pytest/config/__init__.py
+++ b/src/_pytest/config/__init__.py
@@ -1038,7 +1038,7 @@ class Config:
                 mode = "plain"
             else:
                 self._mark_plugins_for_rewrite(hook)
-        _warn_about_missing_assertion(mode)
+        self._warn_about_missing_assertion(mode)
 
     def _mark_plugins_for_rewrite(self, hook) -> None:
         """
@@ -1182,7 +1182,12 @@ class Config:
     def _warn_or_fail_if_strict(self, message: str) -> None:
         if self.known_args_namespace.strict_config:
             fail(message, pytrace=False)
-        sys.stderr.write("WARNING: {}".format(message))
+
+        from _pytest.warnings import _issue_warning_captured
+
+        _issue_warning_captured(
+            PytestConfigWarning(message), self.hook, stacklevel=3,
+        )
 
     def _get_unknown_ini_keys(self) -> List[str]:
         parser_inicfg = self._parser._inidict
@@ -1351,6 +1356,28 @@ class Config:
         """ (deprecated, use getoption(skip=True)) """
         return self.getoption(name, skip=True)
 
+    def _warn_about_missing_assertion(self, mode: str) -> None:
+        if not _assertion_supported():
+            from _pytest.warnings import _issue_warning_captured
+
+            if mode == "plain":
+                warning_text = (
+                    "ASSERTIONS ARE NOT EXECUTED"
+                    " and FAILING TESTS WILL PASS.  Are you"
+                    " using python -O?"
+                )
+            else:
+                warning_text = (
+                    "assertions not in test modules or"
+                    " plugins will be ignored"
+                    " because assert statements are not executed "
+                    "by the underlying Python interpreter "
+                    "(are you using python -O?)\n"
+                )
+            _issue_warning_captured(
+                PytestConfigWarning(warning_text), self.hook, stacklevel=3,
+            )
+
 
 def _assertion_supported() -> bool:
     try:
@@ -1361,24 +1388,6 @@ def _assertion_supported() -> bool:
         return False
 
 
-def _warn_about_missing_assertion(mode) -> None:
-    if not _assertion_supported():
-        if mode == "plain":
-            sys.stderr.write(
-                "WARNING: ASSERTIONS ARE NOT EXECUTED"
-                " and FAILING TESTS WILL PASS.  Are you"
-                " using python -O?"
-            )
-        else:
-            sys.stderr.write(
-                "WARNING: assertions not in test modules or"
-                " plugins will be ignored"
-                " because assert statements are not executed "
-                "by the underlying Python interpreter "
-                "(are you using python -O?)\n"
-            )
-
-
 def create_terminal_writer(
     config: Config, file: Optional[TextIO] = None
 ) -> TerminalWriter:
diff --git a/testing/test_assertion.py b/testing/test_assertion.py
index ae5e75dbf..5c9bb35fa 100644
--- a/testing/test_assertion.py
+++ b/testing/test_assertion.py
@@ -1384,12 +1384,35 @@ def test_exception_handling_no_traceback(testdir):
 
 
 @pytest.mark.skipif("'__pypy__' in sys.builtin_module_names")
-def test_warn_missing(testdir):
+@pytest.mark.parametrize(
+    "cmdline_args, warning_output",
+    [
+        (
+            ["-OO", "-m", "pytest", "-h"],
+            ["warning :*PytestConfigWarning:*assert statements are not executed*"],
+        ),
+        (
+            ["-OO", "-m", "pytest"],
+            [
+                "=*= warnings summary =*=",
+                "*PytestConfigWarning:*assert statements are not executed*",
+            ],
+        ),
+        (
+            ["-OO", "-m", "pytest", "--assert=plain"],
+            [
+                "=*= warnings summary =*=",
+                "*PytestConfigWarning: ASSERTIONS ARE NOT EXECUTED and FAILING TESTS WILL PASS.  "
+                "Are you using python -O?",
+            ],
+        ),
+    ],
+)
+def test_warn_missing(testdir, cmdline_args, warning_output):
     testdir.makepyfile("")
-    result = testdir.run(sys.executable, "-OO", "-m", "pytest", "-h")
-    result.stderr.fnmatch_lines(["*WARNING*assert statements are not executed*"])
-    result = testdir.run(sys.executable, "-OO", "-m", "pytest")
-    result.stderr.fnmatch_lines(["*WARNING*assert statements are not executed*"])
+
+    result = testdir.run(sys.executable, *cmdline_args)
+    result.stdout.fnmatch_lines(warning_output)
 
 
 def test_recursion_source_decode(testdir):
diff --git a/testing/test_config.py b/testing/test_config.py
index bc0da93a5..3d6590d8e 100644
--- a/testing/test_config.py
+++ b/testing/test_config.py
@@ -164,7 +164,7 @@ class TestParseIni:
         assert result.ret == 0
 
     @pytest.mark.parametrize(
-        "ini_file_text, invalid_keys, stderr_output, exception_text",
+        "ini_file_text, invalid_keys, warning_output, exception_text",
         [
             (
                 """
@@ -174,8 +174,9 @@ class TestParseIni:
           """,
                 ["unknown_ini", "another_unknown_ini"],
                 [
-                    "WARNING: Unknown config ini key: another_unknown_ini",
-                    "WARNING: Unknown config ini key: unknown_ini",
+                    "=*= warnings summary =*=",
+                    "*PytestConfigWarning:*Unknown config ini key: another_unknown_ini",
+                    "*PytestConfigWarning:*Unknown config ini key: unknown_ini",
                 ],
                 "Unknown config ini key: another_unknown_ini",
             ),
@@ -186,7 +187,10 @@ class TestParseIni:
           minversion = 5.0.0
           """,
                 ["unknown_ini"],
-                ["WARNING: Unknown config ini key: unknown_ini"],
+                [
+                    "=*= warnings summary =*=",
+                    "*PytestConfigWarning:*Unknown config ini key: unknown_ini",
+                ],
                 "Unknown config ini key: unknown_ini",
             ),
             (
@@ -221,7 +225,7 @@ class TestParseIni:
         ],
     )
     def test_invalid_ini_keys(
-        self, testdir, ini_file_text, invalid_keys, stderr_output, exception_text
+        self, testdir, ini_file_text, invalid_keys, warning_output, exception_text
     ):
         testdir.makeconftest(
             """
@@ -235,7 +239,7 @@ class TestParseIni:
         assert sorted(config._get_unknown_ini_keys()) == sorted(invalid_keys)
 
         result = testdir.runpytest()
-        result.stderr.fnmatch_lines(stderr_output)
+        result.stdout.fnmatch_lines(warning_output)
 
         if exception_text:
             with pytest.raises(pytest.fail.Exception, match=exception_text):