diff --git a/pyproject.toml b/pyproject.toml
index 4f09c020d..0f644d55b 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -208,6 +208,7 @@ exclude = [
 ]
 check_untyped_defs = true
 disallow_incomplete_defs = true
+disallow_untyped_defs = true
 python_version = "3.10"
 show_column_numbers = true
 show_error_context = true
@@ -276,6 +277,12 @@ module = [
 ]
 ignore_missing_imports = true
 
+[[tool.mypy.overrides]]
+module = [
+    "tests.*",
+]
+disallow_untyped_defs = false
+
 [tool.pytest.ini_options]
 minversion = "6.0"
 addopts = [
diff --git a/sphinx/environment/__init__.py b/sphinx/environment/__init__.py
index f99fa8489..279e21586 100644
--- a/sphinx/environment/__init__.py
+++ b/sphinx/environment/__init__.py
@@ -124,7 +124,7 @@ if TYPE_CHECKING:
         def __getitem__(self, key: Literal["todo"]) -> TodoDomain: ...  # NoQA: E704
         @overload
         def __getitem__(self, key: str) -> Domain: ...  # NoQA: E704
-        def __getitem__(self, key): raise NotImplementedError  # NoQA: E704
+        def __getitem__(self, _key: str) -> Domain: raise NotImplementedError  # NoQA: E704
         def __setitem__(  # NoQA: E301,E704
             self, key: str, value: Domain,
         ) -> NoReturn: raise NotImplementedError
diff --git a/sphinx/ext/autodoc/__init__.py b/sphinx/ext/autodoc/__init__.py
index c87562619..1c62d9c9f 100644
--- a/sphinx/ext/autodoc/__init__.py
+++ b/sphinx/ext/autodoc/__init__.py
@@ -1381,7 +1381,7 @@ class FunctionDocumenter(DocstringSignatureMixin, ModuleLevelDocumenter):  # typ
         if len(sig.parameters) == 0:
             return None
 
-        def dummy():  # NoQA: ANN202
+        def dummy():  # type: ignore[no-untyped-def]  # NoQA: ANN202
             pass
 
         params = list(sig.parameters.values())
@@ -2277,7 +2277,7 @@ class MethodDocumenter(DocstringSignatureMixin, ClassLevelDocumenter):  # type:
         if len(sig.parameters) == 1:
             return None
 
-        def dummy():  # NoQA: ANN202
+        def dummy():  # type: ignore[no-untyped-def]  # NoQA: ANN202
             pass
 
         params = list(sig.parameters.values())
diff --git a/sphinx/search/__init__.py b/sphinx/search/__init__.py
index 77eadb832..cf73e377f 100644
--- a/sphinx/search/__init__.py
+++ b/sphinx/search/__init__.py
@@ -487,7 +487,7 @@ class IndexBuilder:
         self._index_entries[docname] = sorted(_index_entries)
 
     def _word_collector(self, doctree: nodes.document) -> WordStore:
-        def _visit_nodes(node):
+        def _visit_nodes(node: nodes.Node) -> None:
             if isinstance(node, nodes.comment):
                 return
             elif isinstance(node, nodes.raw):
diff --git a/utils/babel_runner.py b/utils/babel_runner.py
index d8d9a386a..ef2d63ed9 100644
--- a/utils/babel_runner.py
+++ b/utils/babel_runner.py
@@ -246,7 +246,7 @@ def run_compile() -> None:
             _write_pr_body_line(f'* {locale}: {err_count} error{s}')
 
 
-def _get_logger():
+def _get_logger() -> logging.Logger:
     log = logging.getLogger('babel')
     log.setLevel(logging.INFO)
     handler = logging.StreamHandler()