diff --git a/homeassistant/helpers/template.py b/homeassistant/helpers/template.py
index 7c1113bdda8..8d837bc9bc6 100644
--- a/homeassistant/helpers/template.py
+++ b/homeassistant/helpers/template.py
@@ -1836,14 +1836,24 @@ def forgiving_as_timestamp(value, default=_SENTINEL):
         return default
 
 
-def as_datetime(value):
+def as_datetime(value: Any, default: Any = _SENTINEL) -> Any:
     """Filter and to convert a time string or UNIX timestamp to datetime object."""
     try:
         # Check for a valid UNIX timestamp string, int or float
         timestamp = float(value)
         return dt_util.utc_from_timestamp(timestamp)
-    except ValueError:
-        return dt_util.parse_datetime(value)
+    except (ValueError, TypeError):
+        # Try to parse datetime string to datetime object
+        try:
+            return dt_util.parse_datetime(value, raise_on_error=True)
+        except (ValueError, TypeError):
+            if default is _SENTINEL:
+                # Return None on string input
+                # to ensure backwards compatibility with HA Core 2024.1 and before.
+                if isinstance(value, str):
+                    return None
+                raise_no_default("as_datetime", value)
+            return default
 
 
 def as_timedelta(value: str) -> timedelta | None:
diff --git a/tests/helpers/test_template.py b/tests/helpers/test_template.py
index 90af925ddca..955cd2fd65e 100644
--- a/tests/helpers/test_template.py
+++ b/tests/helpers/test_template.py
@@ -1151,7 +1151,6 @@ def test_as_datetime(hass: HomeAssistant, input) -> None:
     expected = dt_util.parse_datetime(input)
     if expected is not None:
         expected = str(expected)
-
     assert (
         template.Template(f"{{{{ as_datetime('{input}') }}}}", hass).async_render()
         == expected
@@ -1162,34 +1161,64 @@ def test_as_datetime(hass: HomeAssistant, input) -> None:
     )
 
 
-def test_as_datetime_from_timestamp(hass: HomeAssistant) -> None:
-    """Test converting a UNIX timestamp to a date object."""
-    tests = [
+@pytest.mark.parametrize(
+    ("input", "output"),
+    [
         (1469119144, "2016-07-21 16:39:04+00:00"),
         (1469119144.0, "2016-07-21 16:39:04+00:00"),
         (-1, "1969-12-31 23:59:59+00:00"),
-    ]
-    for input, output in tests:
-        # expected = dt_util.parse_datetime(input)
-        if output is not None:
-            output = str(output)
+    ],
+)
+def test_as_datetime_from_timestamp(
+    hass: HomeAssistant,
+    input: int | float,
+    output: str,
+) -> None:
+    """Test converting a UNIX timestamp to a date object."""
+    assert (
+        template.Template(f"{{{{ as_datetime({input}) }}}}", hass).async_render()
+        == output
+    )
+    assert (
+        template.Template(f"{{{{ {input} | as_datetime }}}}", hass).async_render()
+        == output
+    )
+    assert (
+        template.Template(f"{{{{ as_datetime('{input}') }}}}", hass).async_render()
+        == output
+    )
+    assert (
+        template.Template(f"{{{{ '{input}' | as_datetime }}}}", hass).async_render()
+        == output
+    )
 
-        assert (
-            template.Template(f"{{{{ as_datetime({input}) }}}}", hass).async_render()
-            == output
-        )
-        assert (
-            template.Template(f"{{{{ {input} | as_datetime }}}}", hass).async_render()
-            == output
-        )
-        assert (
-            template.Template(f"{{{{ as_datetime('{input}') }}}}", hass).async_render()
-            == output
-        )
-        assert (
-            template.Template(f"{{{{ '{input}' | as_datetime }}}}", hass).async_render()
-            == output
-        )
+
+@pytest.mark.parametrize(
+    ("input", "default", "output"),
+    [
+        (1469119144, 123, "2016-07-21 16:39:04+00:00"),
+        ('"invalid"', ["default output"], ["default output"]),
+        (["a", "list"], 0, 0),
+        ({"a": "dict"}, None, None),
+    ],
+)
+def test_as_datetime_default(
+    hass: HomeAssistant, input: Any, default: Any, output: str
+) -> None:
+    """Test invalid input and return default value."""
+
+    assert (
+        template.Template(
+            f"{{{{ as_datetime({input}, default={default}) }}}}", hass
+        ).async_render()
+        == output
+    )
+    assert (
+        template.Template(
+            f"{{{{ {input} | as_datetime({default}) }}}}", hass
+        ).async_render()
+        == output
+    )
 
 
 def test_as_local(hass: HomeAssistant) -> None: