diff --git a/CHANGES b/CHANGES
index 469f1972cc1..8b1d5b92484 100644
--- a/CHANGES
+++ b/CHANGES
@@ -20,6 +20,8 @@ Bugs fixed
 ----------
 
 * #7461: py domain: fails with IndexError for empty tuple in type annotation
+* #7510: py domain: keyword-only arguments are documented as having a default of
+  None
 * #7418: std domain: :rst:role:`term` role could not match case-insensitively
 * #7461: autodoc: empty tuple in type annotation is not shown correctly
 * #7479: autodoc: Sphinx builds has been slower since 3.0.0 on mocking
diff --git a/sphinx/util/inspect.py b/sphinx/util/inspect.py
index 64c1568f61c..a0ea52047e9 100644
--- a/sphinx/util/inspect.py
+++ b/sphinx/util/inspect.py
@@ -527,7 +527,7 @@ def signature_from_str(signature: str) -> inspect.Signature:
                                 annotation=annotation))
 
     for i, arg in enumerate(args.kwonlyargs):
-        default = ast_unparse(args.kw_defaults[i])
+        default = ast_unparse(args.kw_defaults[i]) or Parameter.empty
         annotation = ast_unparse(arg.annotation) or Parameter.empty
         params.append(Parameter(arg.arg, Parameter.KEYWORD_ONLY, default=default,
                                 annotation=annotation))
diff --git a/tests/test_domain_py.py b/tests/test_domain_py.py
index e330ba6349c..d9d61db1429 100644
--- a/tests/test_domain_py.py
+++ b/tests/test_domain_py.py
@@ -344,9 +344,7 @@ def test_pyfunction_signature_full_py38(app):
     doctree = restructuredtext.parse(app, text)
     assert_node(doctree[1][0][1],
                 [desc_parameterlist, ([desc_parameter, nodes.inline, "*"],
-                                      [desc_parameter, ([desc_sig_name, "a"],
-                                                        [desc_sig_operator, "="],
-                                                        [nodes.inline, "None"])])])
+                                      [desc_parameter, desc_sig_name, "a"])])
 
     # case: separator in the middle
     text = ".. py:function:: hello(a, /, b, *, c)"
@@ -356,9 +354,7 @@ def test_pyfunction_signature_full_py38(app):
                                       [desc_parameter, desc_sig_operator, "/"],
                                       [desc_parameter, desc_sig_name, "b"],
                                       [desc_parameter, desc_sig_operator, "*"],
-                                      [desc_parameter, ([desc_sig_name, "c"],
-                                                        [desc_sig_operator, "="],
-                                                        [nodes.inline, "None"])])])
+                                      [desc_parameter, desc_sig_name, "c"])])
 
     # case: separator in the middle (2)
     text = ".. py:function:: hello(a, /, *, b)"
@@ -367,9 +363,7 @@ def test_pyfunction_signature_full_py38(app):
                 [desc_parameterlist, ([desc_parameter, desc_sig_name, "a"],
                                       [desc_parameter, desc_sig_operator, "/"],
                                       [desc_parameter, desc_sig_operator, "*"],
-                                      [desc_parameter, ([desc_sig_name, "b"],
-                                                        [desc_sig_operator, "="],
-                                                        [nodes.inline, "None"])])])
+                                      [desc_parameter, desc_sig_name, "b"])])
 
     # case: separator at tail
     text = ".. py:function:: hello(a, /)"
diff --git a/tests/test_util_inspect.py b/tests/test_util_inspect.py
index 627d20f54e2..ff1074702c6 100644
--- a/tests/test_util_inspect.py
+++ b/tests/test_util_inspect.py
@@ -317,6 +317,15 @@ def test_signature_from_str_complex_annotations():
     assert sig.return_annotation == 'Callable[[int, int], int]'
 
 
+def test_signature_from_str_kwonly_args():
+    sig = inspect.signature_from_str('(a, *, b)')
+    assert list(sig.parameters.keys()) == ['a', 'b']
+    assert sig.parameters['a'].kind == Parameter.POSITIONAL_OR_KEYWORD
+    assert sig.parameters['a'].default == Parameter.empty
+    assert sig.parameters['b'].kind == Parameter.KEYWORD_ONLY
+    assert sig.parameters['b'].default == Parameter.empty
+
+
 @pytest.mark.skipif(sys.version_info < (3, 8),
                     reason='python-3.8 or above is required')
 def test_signature_from_str_positionaly_only_args():