diff --git a/Lib/site.py b/Lib/site.py
index 873c562d890a3b..294a4d6f641c39 100644
--- a/Lib/site.py
+++ b/Lib/site.py
@@ -510,6 +510,12 @@ def _exec_imports(self):
             # generated by setuptools use: sys._getframe(1).f_locals['sitedir'].
             sitedir = os.path.dirname(filename)
 
+            # Inject 'fullname' local variable in the current frame for
+            # compatibility with Python 3.14, where the path of the ".pth"
+            # file being processed was available to import lines under that
+            # name.
+            fullname = filename
+
             # Given "/path/to/foo.pth", check whether "/path/to/foo.start" was
             # registered in this same batch.
             name, dot, pth = filename.rpartition(".")
diff --git a/Lib/test/test_site.py b/Lib/test/test_site.py
index bcb51ed7d8476a..fc5e6369eacaad 100644
--- a/Lib/test/test_site.py
+++ b/Lib/test/test_site.py
@@ -247,6 +247,23 @@ def test_sitedir_variable(self):
         sitedir = stdout.getvalue().rstrip()
         self.assertEqual(sitedir, pth_dir)
 
+    def test_fullname_variable(self):
+        # gh-156404: ".pth" files generated before Python 3.15 read the path
+        # of the file being processed from the frame executing the import
+        # line, under the name "fullname". For example, "wheel-axle"
+        # generates: "import wheel_axle.runtime;
+        # wheel_axle.runtime.finalize(fullname);"
+        code = '; '.join((
+            "import sys",
+            "print(fullname)",
+            "print(filename)",
+        ))
+        pth_dir, pth_fn = self.make_pth(code)
+        with support.captured_stdout() as stdout:
+            site.addpackage(pth_dir, pth_fn, set())
+        expected = os.path.join(pth_dir, pth_fn)
+        self.assertEqual(stdout.getvalue().splitlines(), [expected, expected])
+
     # This tests _getuserbase, hence the double underline
     # to distinguish from a test for getuserbase
     def test__getuserbase(self):
diff --git a/Misc/NEWS.d/next/Library/2026-08-26-08-20-07.gh-issue-156404.2gP33I.rst b/Misc/NEWS.d/next/Library/2026-08-26-08-20-07.gh-issue-156404.2gP33I.rst
new file mode 100644
index 00000000000000..475d09c764aa3b
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-08-26-08-20-07.gh-issue-156404.2gP33I.rst
@@ -0,0 +1,3 @@
+Restore compatibility with :file:`.pth` files generated before Python 3.15 in
+the :mod:`site` module. Inject the ``fullname`` variable in the frame which
+executes pth code.