diff --git a/CHANGELOG.md b/CHANGELOG.md
index fba0006..5f0df0f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,10 @@ These breaking changes apply to Python JSONPath in its default configuration. We
 - Slice selector indexes and step now follow the specification. Previously leading zeros and negative zero were allowed, now they raise a `JSONPathSyntaxError`.
 - Whitespace is no longer allowed between a dot (`.` or `..`) and a name when using shorthand notation for the name selector. Whitespace before the dot oor double dot is OK.
 
+**JSONPath function extension changes**
+
+- The non-standard `keys()` function extension has been reimplemented. It used to be a simple Python function, `jsonpath.function_extensions.keys`. Now it is a "well-typed" class, `jsonpath.function_extensions.Keys`. See the [filter functions](https://jg-rp.github.io/python-jsonpath/functions/#keys) documentation.
+
 **JSONPath features**
 
 - Added the [Keys filter selector](https://jg-rp.github.io/python-jsonpath/syntax/#keys-filter-selector).
diff --git a/docs/functions.md b/docs/functions.md
index 10504cd..dddf339 100644
--- a/docs/functions.md
+++ b/docs/functions.md
@@ -47,6 +47,34 @@ And `is()` is an alias for `isinstance()`:
 $.categories[?is(@.length, 'number')]
 ```
 
+## `keys()`
+
+**_New in version 2.0.0_**
+
+```
+keys(value: object) -> Tuple[str, ...] | Nothing
+```
+
+Return a list of keys from an object/mapping. If `value` does not have a `keys()` method, the special _Nothing_ value is returned.
+
+!!! note
+
+    `keys()` is not registered with the default JSONPath environment. The [keys selector](syntax.md#keys-selector) and [keys filter selector](syntax.md#keys-filter-selector) are usually the better choice when strict compliance with the specification is not needed.
+
+    You can register `keys()` with your JSONPath environment like this:
+
+    ```python
+    from jsonpath import JSONPathEnvironment
+    from jsonpath import function_extensions
+
+    env = JSONPathEnvironment()
+    env.function_extensions["keys"] = function_extensions.Keys()
+    ```
+
+```
+$.some[?'thing' in keys(@)]
+```
+
 ## `length()`
 
 ```text
diff --git a/jsonpath/function_extensions/__init__.py b/jsonpath/function_extensions/__init__.py
index cd7e8c9..32848a3 100644
--- a/jsonpath/function_extensions/__init__.py
+++ b/jsonpath/function_extensions/__init__.py
@@ -4,7 +4,7 @@
 from .filter_function import FilterFunction
 from .count import Count
 from .is_instance import IsInstance
-from .keys import keys
+from .keys import Keys
 from .length import Length
 from .match import Match
 from .search import Search
@@ -16,7 +16,7 @@
     "ExpressionType",
     "FilterFunction",
     "IsInstance",
-    "keys",
+    "Keys",
     "Length",
     "Match",
     "Search",
diff --git a/jsonpath/function_extensions/keys.py b/jsonpath/function_extensions/keys.py
index bf9ef8c..5a22644 100644
--- a/jsonpath/function_extensions/keys.py
+++ b/jsonpath/function_extensions/keys.py
@@ -1,12 +1,31 @@
-"""The built-in `keys` function extension."""
+"""The `keys` JSONPath filter function."""
+
 from typing import Mapping
-from typing import Optional
 from typing import Tuple
+from typing import Union
+
+from jsonpath.filter import UNDEFINED
+from jsonpath.filter import _Undefined
+
+from .filter_function import ExpressionType
+from .filter_function import FilterFunction
+
+
+class Keys(FilterFunction):
+    """The `keys` JSONPath filter function."""
+
+    arg_types = [ExpressionType.VALUE]
+    return_type = ExpressionType.VALUE
 
+    def __call__(
+        self, value: Mapping[str, object]
+    ) -> Union[Tuple[str, ...], _Undefined]:
+        """Return a tuple of keys in `value`.
 
-def keys(obj: Mapping[str, object]) -> Optional[Tuple[str, ...]]:
-    """Return an object's keys, or `None` if the object has no _keys_ method."""
-    try:
-        return tuple(obj.keys())
-    except AttributeError:
-        return None
+        If `value` does not have a `keys()` method, the special _Nothing_ value
+        is returned.
+        """
+        try:
+            return tuple(value.keys())
+        except AttributeError:
+            return UNDEFINED
diff --git a/tests/test_keys_function.py b/tests/test_keys_function.py
new file mode 100644
index 0000000..afae9b8
--- /dev/null
+++ b/tests/test_keys_function.py
@@ -0,0 +1,62 @@
+import dataclasses
+import operator
+from typing import Any
+from typing import Mapping
+from typing import Sequence
+from typing import Union
+
+import pytest
+
+from jsonpath import JSONPathEnvironment
+from jsonpath import function_extensions
+
+
+@dataclasses.dataclass
+class Case:
+    description: str
+    path: str
+    data: Union[Sequence[Any], Mapping[str, Any]]
+    want: Union[Sequence[Any], Mapping[str, Any]]
+
+
+SOME_OBJECT = object()
+
+TEST_CASES = [
+    Case(
+        description="value in keys of an object",
+        path="$.some[?'thing' in keys(@)]",
+        data={"some": [{"thing": "foo"}]},
+        want=[{"thing": "foo"}],
+    ),
+    Case(
+        description="value not in keys of an object",
+        path="$.some[?'else' in keys(@)]",
+        data={"some": [{"thing": "foo"}]},
+        want=[],
+    ),
+    Case(
+        description="keys of an array",
+        path="$[?'thing' in keys(@)]",
+        data={"some": [{"thing": "foo"}]},
+        want=[],
+    ),
+    Case(
+        description="keys of an string value",
+        path="$some[0].thing[?'else' in keys(@)]",
+        data={"some": [{"thing": "foo"}]},
+        want=[],
+    ),
+]
+
+
+@pytest.fixture()
+def env() -> JSONPathEnvironment:
+    _env = JSONPathEnvironment()
+    _env.function_extensions["keys"] = function_extensions.Keys()
+    return _env
+
+
+@pytest.mark.parametrize("case", TEST_CASES, ids=operator.attrgetter("description"))
+def test_isinstance_function(env: JSONPathEnvironment, case: Case) -> None:
+    path = env.compile(case.path)
+    assert path.findall(case.data) == case.want