diff --git a/Lib/encodings/__init__.py b/Lib/encodings/__init__.py
index 18f5da70caf..169c48324f2 100644
--- a/Lib/encodings/__init__.py
+++ b/Lib/encodings/__init__.py
@@ -26,10 +26,11 @@
 
 (c) Copyright CNRI, All Rights Reserved. NO WARRANTY.
 
-"""#"
+"""
 
 import codecs
 import sys
+from _codecs import _normalize_encoding
 from . import aliases
 
 _cache = {}
@@ -62,18 +63,7 @@ def normalize_encoding(encoding):
             "Support for non-ascii encoding names will be removed in 3.17",
             DeprecationWarning, stacklevel=2)
 
-    chars = []
-    punct = False
-    for c in encoding:
-        if c.isalnum() or c == '.':
-            if punct and chars:
-                chars.append('_')
-            if c.isascii():
-                chars.append(c)
-            punct = False
-        else:
-            punct = True
-    return ''.join(chars)
+    return _normalize_encoding(encoding)
 
 def search_function(encoding):
 
diff --git a/crates/vm/src/stdlib/_codecs.rs b/crates/vm/src/stdlib/_codecs.rs
index a17c025f50c..23e92f84797 100644
--- a/crates/vm/src/stdlib/_codecs.rs
+++ b/crates/vm/src/stdlib/_codecs.rs
@@ -20,6 +20,36 @@ mod _codecs {
         function::ArgBytesLike,
     };
 
+    #[derive(FromArgs)]
+    struct NormalizeEncodingArgs {
+        #[pyarg(any)]
+        encoding: PyStrRef,
+    }
+
+    #[pyfunction]
+    fn _normalize_encoding(args: NormalizeEncodingArgs, vm: &VirtualMachine) -> PyResult<String> {
+        let encoding = args.encoding;
+        // CPython converts the entire input to UTF-8 before scanning for NUL.
+        if !encoding.is_utf8() {
+            let ctx = PyEncodeContext::new(encodings::utf8::ENCODING_NAME, &encoding, vm);
+            encodings::utf8::encode(ctx, &encodings::errors::Strict)?;
+        }
+        let mut normalized = String::with_capacity(encoding.byte_len());
+        let mut punct = false;
+        for byte in encoding.as_bytes().iter().copied().take_while(|b| *b != 0) {
+            if byte.is_ascii_alphanumeric() || byte == b'.' {
+                if punct && !normalized.is_empty() {
+                    normalized.push('_');
+                }
+                normalized.push(char::from(byte));
+                punct = false;
+            } else {
+                punct = true;
+            }
+        }
+        Ok(normalized)
+    }
+
     #[pyfunction]
     fn register(search_function: PyObjectRef, vm: &VirtualMachine) -> PyResult<()> {
         vm.state.codec_registry.register(search_function, vm)
diff --git a/extra_tests/snippets/stdlib_codecs_wide.py b/extra_tests/snippets/stdlib_codecs_wide.py
index 0c45cc91cca..bd8307ffcfb 100644
--- a/extra_tests/snippets/stdlib_codecs_wide.py
+++ b/extra_tests/snippets/stdlib_codecs_wide.py
@@ -64,3 +64,79 @@ def record_unknown_encoding(name):
             assert seen_encoding_names[-1] == normalized
 finally:
     codecs.unregister(record_unknown_encoding)
+
+
+# The encodings helper preserves case and dots, unlike registry lookup keys.
+import encodings
+import warnings
+
+
+class EncodingName(str):
+    pass
+
+
+for encoding, expected in (
+    ("", ""),
+    ("UTF-8", "UTF_8"),
+    ("  .UTF--8.  ", ".UTF_8."),
+    ("---", ""),
+    ("A__-- B", "A_B"),
+    ("AéB", "A_B"),
+    ("éUTF-8Ω", "UTF_8"),
+    ("A😀B", "A_B"),
+    ("UTF-8\x00ignored", "UTF_8"),
+    ("\x00UTF-8", ""),
+):
+    with warnings.catch_warnings(record=True) as caught:
+        warnings.simplefilter("always")
+        assert _codecs._normalize_encoding(encoding) == expected
+        assert _codecs._normalize_encoding(encoding=EncodingName(encoding)) == expected
+        assert not caught
+
+for encoding in (b"UTF-8", bytearray(b"UTF-8"), None, 1):
+    try:
+        _codecs._normalize_encoding(encoding)
+    except TypeError:
+        pass
+    else:
+        assert False, "non-string encoding was accepted"
+
+# UTF-8 validation precedes scanning, including a surrogate after NUL.
+for encoding in ("A\ud800B", "A\x00\ud800B"):
+    try:
+        _codecs._normalize_encoding(encoding)
+    except UnicodeEncodeError as error:
+        assert error.encoding == "utf-8"
+        assert error.object == encoding
+        assert error.start == encoding.index("\ud800")
+        assert error.end == error.start + 1
+    else:
+        assert False, "surrogate encoding was accepted"
+
+with warnings.catch_warnings(record=True) as caught:
+    warnings.simplefilter("always")
+    assert encodings.normalize_encoding(b"UTF-8") == "UTF_8"
+    assert not caught
+    assert encodings.normalize_encoding("AéB") == "A_B"
+    assert len(caught) == 1
+    assert caught[0].category is DeprecationWarning
+
+try:
+    encodings.normalize_encoding(b"UTF-\xff")
+except UnicodeDecodeError:
+    pass
+else:
+    assert False, "non-ASCII bytes encoding was accepted"
+
+for args, kwargs in (
+    ((), {}),
+    (("UTF-8", "ASCII"), {}),
+    (("UTF-8",), {"encoding": "ASCII"}),
+    ((), {"name": "UTF-8"}),
+):
+    try:
+        _codecs._normalize_encoding(*args, **kwargs)
+    except TypeError:
+        pass
+    else:
+        assert False, "invalid normalization arguments were accepted"