diff --git a/Lib/test/test_wsgiref.py b/Lib/test/test_wsgiref.py
index 484a2db1637861c..e0225bdec2eb493 100644
--- a/Lib/test/test_wsgiref.py
+++ b/Lib/test/test_wsgiref.py
@@ -592,6 +592,19 @@ def testMappingInterface(self):
         self.assertEqual(h["foo"],"baz")
         self.assertEqual(h["zoo"],"whee")
 
+    def testSetItemValidationFailureKeepsOldValue(self):
+        # gh-158225: __setitem__ must not delete the old header
+        # before validating the new name and value.
+        for bad_value, exc_type in [
+            ("bad\x00value", ValueError),
+            (123, AssertionError),
+        ]:
+            with self.subTest(bad_value=bad_value):
+                h = Headers([("Content-Type", "text/html")])
+                with self.assertRaises(exc_type):
+                    h["Content-Type"] = bad_value
+                self.assertEqual(h["Content-Type"], "text/html")
+
     def testRequireList(self):
         self.assertRaises(TypeError, Headers, "foo")
 
diff --git a/Lib/wsgiref/headers.py b/Lib/wsgiref/headers.py
index eb6ea6a412dcc90..a27eec3a7dbb363 100644
--- a/Lib/wsgiref/headers.py
+++ b/Lib/wsgiref/headers.py
@@ -59,11 +59,12 @@ def __len__(self):
 
     def __setitem__(self, name, val):
         """Set the value of a header."""
+        name = self._convert_string_type(name, name=True)
+        val = self._convert_string_type(val, name=False)
         del self[name]
-        self._headers.append(
-            (self._convert_string_type(name, name=True), self._convert_string_type(val, name=False)))
+        self._headers.append((name, val))
 
-    def __delitem__(self,name):
+    def __delitem__(self, name):
         """Delete all occurrences of a header, if present.
 
         Does *not* raise an exception if the header is missing.
diff --git a/Misc/NEWS.d/next/Library/2026-09-26-17-24-55.gh-issue-158225.AbCdEf.rst b/Misc/NEWS.d/next/Library/2026-09-26-17-24-55.gh-issue-158225.AbCdEf.rst
new file mode 100644
index 000000000000000..48121c5e4ac2bb7
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-09-26-17-24-55.gh-issue-158225.AbCdEf.rst
@@ -0,0 +1,3 @@
+Fix :class:`wsgiref.headers.Headers` so that ``__setitem__`` keeps the
+existing header value when validation of the new value fails. Patch by
+Tony Leung.