diff --git a/AUTHORS b/AUTHORS
index a76c0a632..e1b195b9a 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -109,6 +109,7 @@ Gabriel Reis
 Gene Wood
 George Kussumoto
 Georgy Dyuldin
+Gleb Nikonorov
 Graham Horler
 Greg Price
 Gregory Lee
diff --git a/changelog/6856.feature.rst b/changelog/6856.feature.rst
new file mode 100644
index 000000000..36892fa21
--- /dev/null
+++ b/changelog/6856.feature.rst
@@ -0,0 +1,3 @@
+A warning is now shown when an unknown key is read from a config INI file.
+
+The `--strict-config` flag has been added to treat these warnings as errors.
diff --git a/src/_pytest/config/__init__.py b/src/_pytest/config/__init__.py
index bb5034ab1..5fc23716d 100644
--- a/src/_pytest/config/__init__.py
+++ b/src/_pytest/config/__init__.py
@@ -1030,6 +1030,7 @@ class Config:
         self.known_args_namespace = ns = self._parser.parse_known_args(
             args, namespace=copy.copy(self.option)
         )
+        self._validatekeys()
         if self.known_args_namespace.confcutdir is None and self.inifile:
             confcutdir = py.path.local(self.inifile).dirname
             self.known_args_namespace.confcutdir = confcutdir
@@ -1072,6 +1073,17 @@ class Config:
                     )
                 )
 
+    def _validatekeys(self):
+        for key in self._get_unknown_ini_keys():
+            message = "Unknown config ini key: {}\n".format(key)
+            if self.known_args_namespace.strict_config:
+                fail(message, pytrace=False)
+            sys.stderr.write("WARNING: {}".format(message))
+
+    def _get_unknown_ini_keys(self) -> List[str]:
+        parser_inicfg = self._parser._inidict
+        return [name for name in self.inicfg if name not in parser_inicfg]
+
     def parse(self, args: List[str], addopts: bool = True) -> None:
         # parse given cmdline arguments into this config object.
         assert not hasattr(
diff --git a/src/_pytest/main.py b/src/_pytest/main.py
index de7e16744..4eb47be2c 100644
--- a/src/_pytest/main.py
+++ b/src/_pytest/main.py
@@ -70,6 +70,11 @@ def pytest_addoption(parser):
         default=0,
         help="exit after first num failures or errors.",
     )
+    group._addoption(
+        "--strict-config",
+        action="store_true",
+        help="invalid ini keys for the `pytest` section of the configuration file raise errors.",
+    )
     group._addoption(
         "--strict-markers",
         "--strict",
diff --git a/testing/test_config.py b/testing/test_config.py
index 17385dc17..6a08e93f3 100644
--- a/testing/test_config.py
+++ b/testing/test_config.py
@@ -147,6 +147,70 @@ class TestParseIni:
         result = testdir.inline_run("--confcutdir=.")
         assert result.ret == 0
 
+    @pytest.mark.parametrize(
+        "ini_file_text, invalid_keys, stderr_output, exception_text",
+        [
+            (
+                """
+          [pytest]
+          unknown_ini = value1
+          another_unknown_ini = value2
+          """,
+                ["unknown_ini", "another_unknown_ini"],
+                [
+                    "WARNING: Unknown config ini key: unknown_ini",
+                    "WARNING: Unknown config ini key: another_unknown_ini",
+                ],
+                "Unknown config ini key: unknown_ini",
+            ),
+            (
+                """
+          [pytest]
+          unknown_ini = value1
+          minversion = 5.0.0
+          """,
+                ["unknown_ini"],
+                ["WARNING: Unknown config ini key: unknown_ini"],
+                "Unknown config ini key: unknown_ini",
+            ),
+            (
+                """
+          [some_other_header]
+          unknown_ini = value1
+          [pytest]
+          minversion = 5.0.0
+          """,
+                [],
+                [],
+                "",
+            ),
+            (
+                """
+          [pytest]
+          minversion = 5.0.0
+          """,
+                [],
+                [],
+                "",
+            ),
+        ],
+    )
+    def test_invalid_ini_keys(
+        self, testdir, ini_file_text, invalid_keys, stderr_output, exception_text
+    ):
+        testdir.tmpdir.join("pytest.ini").write(textwrap.dedent(ini_file_text))
+        config = testdir.parseconfig()
+        assert config._get_unknown_ini_keys() == invalid_keys, str(
+            config._get_unknown_ini_keys()
+        )
+
+        result = testdir.runpytest()
+        result.stderr.fnmatch_lines(stderr_output)
+
+        if stderr_output:
+            with pytest.raises(pytest.fail.Exception, match=exception_text):
+                testdir.runpytest("--strict-config")
+
 
 class TestConfigCmdlineParsing:
     def test_parsing_again_fails(self, testdir):