diff --git a/mkdocs_rss_plugin/util.py b/mkdocs_rss_plugin/util.py
index 20c88794..35264c38 100644
--- a/mkdocs_rss_plugin/util.py
+++ b/mkdocs_rss_plugin/util.py
@@ -560,7 +560,8 @@ def guess_locale(mkdocs_config: Config) -> str or None:
 
         # Some themes implement a locale or a language setting
         if "theme" in mkdocs_config and "locale" in mkdocs_config.get("theme"):
-            return mkdocs_config.get("theme")._vars.get("locale")
+            locale = mkdocs_config.get("theme")._vars.get("locale")
+            return f"{locale.language}-{locale.territory}"
         elif "theme" in mkdocs_config and "language" in mkdocs_config.get("theme"):
             return mkdocs_config.get("theme")._vars.get("language")
         else:
diff --git a/tests/fixtures/mkdocs_lang_with_territory.yml b/tests/fixtures/mkdocs_lang_with_territory.yml
new file mode 100644
index 00000000..f765fc60
--- /dev/null
+++ b/tests/fixtures/mkdocs_lang_with_territory.yml
@@ -0,0 +1,12 @@
+site_name: Test RSS Plugin
+site_description: Test a language code with territory
+site_url: https://guts.github.io/mkdocs-rss-plugin
+
+use_directory_urls: true
+
+plugins:
+    - rss
+
+theme:
+    name: mkdocs
+    locale: en_US
diff --git a/tests/test_build.py b/tests/test_build.py
index 16c35289..7f8ca386 100644
--- a/tests/test_build.py
+++ b/tests/test_build.py
@@ -330,6 +330,32 @@ def test_simple_build_item_length_unlimited(self):
                         len(feed_item.description), 150, feed_item.title
                     )
 
+    def test_simple_build_lang_with_territory(self):
+        with tempfile.TemporaryDirectory() as tmpdirname:
+            cli_result = self.build_docs_setup(
+                testproject_path="docs",
+                mkdocs_yml_filepath=Path(
+                    "tests/fixtures/mkdocs_lang_with_territory.yml"
+                ),
+                output_path=tmpdirname,
+                strict=True,
+            )
+
+            if cli_result.exception is not None:
+                e = cli_result.exception
+                logger.debug(format_exception(type(e), e, e.__traceback__))
+
+            self.assertEqual(cli_result.exit_code, 0)
+            self.assertIsNone(cli_result.exception)
+
+            # created items
+            feed_parsed = feedparser.parse(Path(tmpdirname) / "feed_rss_created.xml")
+            self.assertEqual(feed_parsed.feed.get("language"), "en-US")
+
+            # updated items
+            feed_parsed = feedparser.parse(Path(tmpdirname) / "feed_rss_updated.xml")
+            self.assertEqual(feed_parsed.feed.get("language"), "en-US")
+
     def test_simple_build_pretty_print_enabled(self):
         with tempfile.TemporaryDirectory() as tmpdirname:
             cli_result = self.build_docs_setup(