diff --git a/src/packaging/tags.py b/src/packaging/tags.py
index 14bf982b..76d24341 100644
--- a/src/packaging/tags.py
+++ b/src/packaging/tags.py
@@ -120,7 +120,7 @@ def _get_config_var(name: str, warn: bool = False) -> Union[int, str, None]:
 
 
 def _normalize_string(string: str) -> str:
-    return string.replace(".", "_").replace("-", "_")
+    return string.replace(".", "_").replace("-", "_").replace(" ", "_")
 
 
 def _abi3_applies(python_version: PythonVersion) -> bool:
diff --git a/tests/test_tags.py b/tests/test_tags.py
index 7c854735..1ee62711 100644
--- a/tests/test_tags.py
+++ b/tests/test_tags.py
@@ -599,6 +599,13 @@ def test_platform_tags(platform_name, dispatch_func, monkeypatch):
     assert tags.platform_tags() == expected
 
 
+def test_platform_tags_space(monkeypatch):
+    """Ensure spaces in platform tags are normalized to underscores."""
+    monkeypatch.setattr(platform, "system", lambda: "Isilon OneFS")
+    monkeypatch.setattr(sysconfig, "get_platform", lambda: "isilon onefs")
+    assert list(tags.platform_tags()) == ["isilon_onefs"]
+
+
 class TestCPythonABI:
     @pytest.mark.parametrize(
         "py_debug,gettotalrefcount,result",
@@ -770,6 +777,12 @@ def test_platforms_defaults_needs_underscore(self, monkeypatch):
         result = list(tags.cpython_tags((3, 11), abis=["whatever"]))
         assert tags.Tag("cp311", "whatever", "plat1") in result
 
+    def test_platform_name_space_normalization(self, monkeypatch):
+        """Ensure that spaces are translated to underscores in platform names."""
+        monkeypatch.setattr(sysconfig, "get_platform", lambda: "isilon onefs")
+        for tag in tags.cpython_tags():
+            assert " " not in tag.platform
+
     def test_major_only_python_version(self):
         result = list(tags.cpython_tags((3,), ["abi"], ["plat"]))
         assert result == [
@@ -839,9 +852,9 @@ def test__generic_abi_linux_cpython(self, monkeypatch):
         assert tags._generic_abi() == ["cp37m"]
 
     def test__generic_abi_jp(self, monkeypatch):
-        config = {"EXT_SUFFIX": ".return exactly this.so"}
+        config = {"EXT_SUFFIX": ".return_exactly_this.so"}
         monkeypatch.setattr(sysconfig, "get_config_var", config.__getitem__)
-        assert tags._generic_abi() == ["return exactly this"]
+        assert tags._generic_abi() == ["return_exactly_this"]
 
     def test__generic_abi_graal(self, monkeypatch):
         config = {"EXT_SUFFIX": ".graalpy-38-native-x86_64-darwin.so"}
@@ -897,6 +910,12 @@ def test_generic_platforms(self):
         platform = platform.replace(".", "_")
         assert list(tags._generic_platforms()) == [platform]
 
+    def test_generic_platforms_space(self, monkeypatch):
+        """Ensure platform tags normalize spaces to underscores."""
+        platform_ = "isilon onefs"
+        monkeypatch.setattr(sysconfig, "get_platform", lambda: platform_)
+        assert list(tags._generic_platforms()) == [platform_.replace(" ", "_")]
+
     def test_iterator_returned(self):
         result_iterator = tags.generic_tags("sillywalk33", ["abi"], ["plat1", "plat2"])
         assert isinstance(result_iterator, collections.abc.Iterator)