diff --git a/sphinx/util/inspect.py b/sphinx/util/inspect.py
index 0f3f47562..d4928c847 100644
--- a/sphinx/util/inspect.py
+++ b/sphinx/util/inspect.py
@@ -527,10 +527,14 @@ def stringify_signature(sig: inspect.Signature, show_annotation: bool = True,
 def signature_from_str(signature: str) -> inspect.Signature:
     """Create a Signature object from string."""
     module = ast.parse('def func' + signature + ': pass')
-    definition = cast(ast.FunctionDef, module.body[0])  # type: ignore
+    function = cast(ast.FunctionDef, module.body[0])  # type: ignore
 
-    # parameters
-    args = definition.args
+    return signature_from_ast(function)
+
+
+def signature_from_ast(node: ast.FunctionDef) -> inspect.Signature:
+    """Create a Signature object from AST *node*."""
+    args = node.args
     defaults = list(args.defaults)
     params = []
     if hasattr(args, "posonlyargs"):
@@ -580,7 +584,7 @@ def signature_from_str(signature: str) -> inspect.Signature:
         params.append(Parameter(args.kwarg.arg, Parameter.VAR_KEYWORD,
                                 annotation=annotation))
 
-    return_annotation = ast_unparse(definition.returns) or Parameter.empty
+    return_annotation = ast_unparse(node.returns) or Parameter.empty
 
     return inspect.Signature(params, return_annotation=return_annotation)
 
diff --git a/tests/test_util_inspect.py b/tests/test_util_inspect.py
index fa0ff84e1..6a14dc1ac 100644
--- a/tests/test_util_inspect.py
+++ b/tests/test_util_inspect.py
@@ -9,6 +9,7 @@
 """
 
 import _testcapi
+import ast
 import datetime
 import functools
 import sys
@@ -350,6 +351,38 @@ def test_signature_from_str_invalid():
         inspect.signature_from_str('')
 
 
+def test_signature_from_ast():
+    signature = 'def func(a, b, *args, c=0, d="blah", **kwargs): pass'
+    tree = ast.parse(signature)
+    sig = inspect.signature_from_ast(tree.body[0])
+    assert list(sig.parameters.keys()) == ['a', 'b', 'args', 'c', 'd', 'kwargs']
+    assert sig.parameters['a'].name == 'a'
+    assert sig.parameters['a'].kind == Parameter.POSITIONAL_OR_KEYWORD
+    assert sig.parameters['a'].default == Parameter.empty
+    assert sig.parameters['a'].annotation == Parameter.empty
+    assert sig.parameters['b'].name == 'b'
+    assert sig.parameters['b'].kind == Parameter.POSITIONAL_OR_KEYWORD
+    assert sig.parameters['b'].default == Parameter.empty
+    assert sig.parameters['b'].annotation == Parameter.empty
+    assert sig.parameters['args'].name == 'args'
+    assert sig.parameters['args'].kind == Parameter.VAR_POSITIONAL
+    assert sig.parameters['args'].default == Parameter.empty
+    assert sig.parameters['args'].annotation == Parameter.empty
+    assert sig.parameters['c'].name == 'c'
+    assert sig.parameters['c'].kind == Parameter.KEYWORD_ONLY
+    assert sig.parameters['c'].default == '0'
+    assert sig.parameters['c'].annotation == Parameter.empty
+    assert sig.parameters['d'].name == 'd'
+    assert sig.parameters['d'].kind == Parameter.KEYWORD_ONLY
+    assert sig.parameters['d'].default == "'blah'"
+    assert sig.parameters['d'].annotation == Parameter.empty
+    assert sig.parameters['kwargs'].name == 'kwargs'
+    assert sig.parameters['kwargs'].kind == Parameter.VAR_KEYWORD
+    assert sig.parameters['kwargs'].default == Parameter.empty
+    assert sig.parameters['kwargs'].annotation == Parameter.empty
+    assert sig.return_annotation == Parameter.empty
+
+
 def test_safe_getattr_with_default():
     class Foo:
         def __getattr__(self, item):