diff --git a/Lib/encodings/idna.py b/Lib/encodings/idna.py
index bf98f513366b314..ff72eb3b46ac26c 100644
--- a/Lib/encodings/idna.py
+++ b/Lib/encodings/idna.py
@@ -86,7 +86,7 @@ def ToASCII(label):
         raise UnicodeError("label empty or too long")
 
     # Step 5: Check ACE prefix
-    if label.startswith(sace_prefix):
+    if label[:4].lower() == sace_prefix:
         raise UnicodeError("Label starts with ACE prefix")
 
     # Step 6: Encode with PUNYCODE
@@ -119,7 +119,7 @@ def ToUnicode(label):
         except UnicodeError:
             raise UnicodeError("Invalid character in IDN label")
     # Step 3: Check for ACE prefix
-    if not label.startswith(ace_prefix):
+    if not label[:4].lower() == ace_prefix:
         return str(label, "ascii")
 
     # Step 4: Remove ACE prefix
@@ -192,7 +192,7 @@ def decode(self, input, errors='strict'):
             # XXX obviously wrong, see #3232
             input = bytes(input)
 
-        if ace_prefix not in input:
+        if ace_prefix not in input.lower():
             # Fast path
             try:
                 return input.decode('ascii'), len(input)
diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index a7440eea67c1016..d0963589b9c6df8 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -1548,6 +1548,13 @@ def test_builtin_decode(self):
         self.assertEqual(str(b"python.org.", "idna"), "python.org.")
         self.assertEqual(str(b"xn--pythn-mua.org", "idna"), "pyth\xf6n.org")
         self.assertEqual(str(b"xn--pythn-mua.org.", "idna"), "pyth\xf6n.org.")
+        self.assertEqual(str(b"XN--pythn-mua.org.", "idna"), "pyth\xf6n.org.")
+        self.assertEqual(str(b"xN--pythn-mua.org.", "idna"), "pyth\xf6n.org.")
+        self.assertEqual(str(b"Xn--pythn-mua.org.", "idna"), "pyth\xf6n.org.")
+        self.assertEqual(str(b"bugs.xn--pythn-mua.org.", "idna"),
+                         "bugs.pyth\xf6n.org.")
+        self.assertEqual(str(b"bugs.XN--pythn-mua.org.", "idna"),
+                         "bugs.pyth\xf6n.org.")
 
     def test_builtin_encode(self):
         self.assertEqual("python.org".encode("idna"), b"python.org")
diff --git a/Misc/NEWS.d/next/Library/2024-03-13-15-45-54.gh-issue-63283.OToJnG.rst b/Misc/NEWS.d/next/Library/2024-03-13-15-45-54.gh-issue-63283.OToJnG.rst
new file mode 100644
index 000000000000000..bb4c3a4a8d741b7
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-03-13-15-45-54.gh-issue-63283.OToJnG.rst
@@ -0,0 +1,2 @@
+In :mod:`encodings.idna`, any capitalization of the the ACE prefix
+(``xn--``) is now acceptable. Patch by Pepijn de Vos and Zackery Spytz.