diff --git a/Doc/library/xml.etree.elementtree.rst b/Doc/library/xml.etree.elementtree.rst
index a61fb05bf99d87..8bd7b4ada3a592 100644
--- a/Doc/library/xml.etree.elementtree.rst
+++ b/Doc/library/xml.etree.elementtree.rst
@@ -739,9 +739,14 @@ Functions
    *prefix* is a namespace prefix.  *uri* is a namespace uri.  Tags and
    attributes in this namespace will be serialized with the given prefix, if at
    all possible.
+   :exc:`ValueError` is raised if *prefix* is invalid or reserved
+   (``ns`` followed by digits is reserved for the serializer).
 
    .. versionadded:: 3.2
 
+   .. versionchanged:: next
+      Invalid and reserved prefixes are now rejected.
+
 
 .. function:: SubElement(parent, tag, /, attrib={}, **extra)
 
diff --git a/Lib/test/test_xml_etree.py b/Lib/test/test_xml_etree.py
index 899947c1f8e0d7..6f06c2c8f6a351 100644
--- a/Lib/test/test_xml_etree.py
+++ b/Lib/test/test_xml_etree.py
@@ -2848,6 +2848,36 @@ def test_bug_200709_register_namespace(self):
         self.assertEqual(ET.tostring(e),
             b'<dc:title xmlns:dc="http://purl.org/dc/elements/1.1/" />')
 
+    def test_register_namespace_invalid(self):
+        # gh-157518
+        xml_ns = 'http://www.w3.org/XML/1998/namespace'
+        xmlns_ns = 'http://www.w3.org/2000/xmlns/'
+        nsmap = ET.register_namespace._namespace_map
+        saved = dict(nsmap)
+        for prefix, uri in [
+            ('ns0', 'u'), ('ns12', 'u'),
+            ('xml', 'u'), ('foo', xml_ns),
+            ('xmlns', 'u'), ('foo', xmlns_ns),
+            ('a:b', 'u'), ('1', 'u'), ('a b', 'u'), ('a\xa0b', 'u'),
+        ]:
+            with self.subTest(prefix=prefix, uri=uri):
+                with self.assertRaises(ValueError):
+                    ET.register_namespace(prefix, uri)
+                # the registry is not changed
+                self.assertEqual(nsmap, saved)
+        for prefix, uri in [(1, 'u'), (b'a', 'u'), ('a', 1), ('a', None)]:
+            with self.subTest(prefix=prefix, uri=uri):
+                with self.assertRaises(TypeError):
+                    ET.register_namespace(prefix, uri)
+        # the xml prefix can be registered for its namespace
+        ET.register_namespace('xml', xml_ns)
+        self.assertEqual(ET.register_namespace._namespace_map[xml_ns], 'xml')
+        # non-ASCII names are valid
+        self.addCleanup(ET.register_namespace._namespace_map.pop, 'u', None)
+        ET.register_namespace('\xe9', 'u')
+        self.assertEqual(ET.tostring(ET.Element('{u}a'), encoding='unicode'),
+                         '<\xe9:a xmlns:\xe9="u" />')
+
     def test_bug_200709_element_comment(self):
         # Not sure if this can be fixed, really (since the serializer needs
         # ET.Comment, not cET.comment).
diff --git a/Lib/xml/etree/ElementTree.py b/Lib/xml/etree/ElementTree.py
index 0cd04ead801600..10200671634013 100644
--- a/Lib/xml/etree/ElementTree.py
+++ b/Lib/xml/etree/ElementTree.py
@@ -98,6 +98,7 @@
 import contextlib
 import weakref
 
+from .. import is_valid_name
 from . import ElementPath
 
 
@@ -1029,6 +1030,29 @@ def _serialize_text(write, elem):
 }
 
 
+_XML_NAMESPACE = "http://www.w3.org/XML/1998/namespace"
+_XMLNS_NAMESPACE = "http://www.w3.org/2000/xmlns/"
+
+def _check_prefix(prefix, uri):
+    # Check a namespace prefix and the namespace URI bound to it
+    # (see Namespaces in XML 1.0, 3 and 4).  The empty prefix is allowed.
+    if not isinstance(prefix, str) or not isinstance(uri, str):
+        raise TypeError("namespace prefix and URI must be strings")
+    if re.match(r"ns\d+$", prefix):
+        raise ValueError("Prefix format reserved for internal use")
+    if prefix == "xml":
+        if uri != _XML_NAMESPACE:
+            raise ValueError("the 'xml' prefix can only be bound to "
+                             "the XML namespace")
+    elif uri == _XML_NAMESPACE:
+        raise ValueError("the XML namespace can only be bound to "
+                         "the 'xml' prefix")
+    elif prefix == "xmlns" or uri == _XMLNS_NAMESPACE:
+        raise ValueError("the 'xmlns' prefix cannot be bound")
+    elif prefix and (not is_valid_name(prefix) or ':' in prefix):
+        raise ValueError("invalid namespace prefix %r" % (prefix,))
+
+
 def register_namespace(prefix, uri):
     """Register a namespace prefix.
 
@@ -1041,8 +1065,7 @@ def register_namespace(prefix, uri):
     ValueError is raised if prefix is reserved or is invalid.
 
     """
-    if re.match(r"ns\d+$", prefix):
-        raise ValueError("Prefix format reserved for internal use")
+    _check_prefix(prefix, uri)
     for k, v in list(_namespace_map.items()):
         if k == uri or v == prefix:
             del _namespace_map[k]
diff --git a/Misc/NEWS.d/next/Library/2026-09-14-11-00-00.gh-issue-157518.Rk8mTz.rst b/Misc/NEWS.d/next/Library/2026-09-14-11-00-00.gh-issue-157518.Rk8mTz.rst
new file mode 100644
index 00000000000000..6c3512e7fefbac
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-09-14-11-00-00.gh-issue-157518.Rk8mTz.rst
@@ -0,0 +1,4 @@
+:func:`xml.etree.ElementTree.register_namespace` now raises
+:exc:`ValueError` for an invalid prefix, for the reserved ``xmlns`` prefix,
+and for the ``xml`` prefix or the XML namespace bound to each other's wrong
+counterpart, instead of accepting them and serializing invalid XML.