diff --git a/src/packaging/markers.py b/src/packaging/markers.py
index 2d60bdf2..7ac7bb69 100644
--- a/src/packaging/markers.py
+++ b/src/packaging/markers.py
@@ -309,6 +309,12 @@ def evaluate(self, environment: dict[str, str] | None = None) -> bool:
         """
         current_environment = cast("dict[str, str]", default_environment())
         current_environment["extra"] = ""
+        # Work around platform.python_version() returning something that is not PEP 440
+        # compliant for non-tagged Python builds. We preserve default_environment()'s
+        # behavior of returning platform.python_version() verbatim, and leave it to the
+        # caller to provide a syntactically valid version if they want to override it.
+        if current_environment["python_full_version"].endswith("+"):
+            current_environment["python_full_version"] += "local"
         if environment is not None:
             current_environment.update(environment)
             # The API used to allow setting extra to None. We need to handle this
diff --git a/tests/test_markers.py b/tests/test_markers.py
index a2f92b30..775b51fd 100644
--- a/tests/test_markers.py
+++ b/tests/test_markers.py
@@ -8,6 +8,7 @@
 import platform
 import sys
 from typing import cast
+from unittest import mock
 
 import pytest
 
@@ -19,6 +20,7 @@
     default_environment,
     format_full_version,
 )
+from packaging.version import InvalidVersion
 
 VARIABLES = [
     "extra",
@@ -384,3 +386,14 @@ def test_extra_str_normalization(self):
 
         assert str(Marker(lhs)) == f'"{normalized_name}" == extra'
         assert str(Marker(rhs)) == f'extra == "{normalized_name}"'
+
+    def test_python_full_version_untagged_user_provided(self):
+        """A user-provided python_full_version ending with a + fails to parse."""
+        with pytest.raises(InvalidVersion):
+            Marker("python_full_version < '3.12'").evaluate(
+                {"python_full_version": "3.11.1+"}
+            )
+
+    def test_python_full_version_untagged(self):
+        with mock.patch("platform.python_version", return_value="3.11.1+"):
+            assert Marker("python_full_version < '3.12'").evaluate()