diff --git a/Lib/http/client.py b/Lib/http/client.py
index 7ef99e7201c005..7dbdc8e496c13f 100644
--- a/Lib/http/client.py
+++ b/Lib/http/client.py
@@ -186,11 +186,16 @@ def _encode(data, name='data'):
 
 def _strip_ipv6_iface(enc_name: bytes) -> bytes:
     """Remove interface scope from IPv6 address."""
-    enc_name, percent, _ = enc_name.partition(b"%")
-    if percent:
-        assert enc_name.startswith(b'['), enc_name
-        enc_name += b']'
-    return enc_name
+    before, percent, after = enc_name.partition(b"%")
+    if not percent or not before.startswith(b'['):
+        # No '%' in the input, or it's not a bracketed IPv6 literal, so
+        # this isn't an IPv6 zone id -- e.g. RFC 3986 percent-encoding
+        # elsewhere in the netloc, most plausibly in userinfo (required
+        # whenever a username/password contains '@', ':', '/', or '%').
+        # Leave it untouched rather than assuming the '%' we happened to
+        # find is a zone separator.
+        return enc_name
+    return before + b']'
 
 class HTTPMessage(email.message.Message):
 
diff --git a/Misc/NEWS.d/next/Library/2026-09-25-17-43-30.gh-issue-158182.2MgwWD.rst b/Misc/NEWS.d/next/Library/2026-09-25-17-43-30.gh-issue-158182.2MgwWD.rst
new file mode 100644
index 00000000000000..61188bed774fcf
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-09-25-17-43-30.gh-issue-158182.2MgwWD.rst
@@ -0,0 +1,3 @@
+Fix :exc:`AssertionError` in :meth:`http.client.HTTPConnection.putrequest`
+when an absolute-URL request's netloc contains RFC 3986 percent-encoding
+outside of an IPv6 zone identifier, such as percent-encoded userinfo.