Joe Keyes ran in a hat-trick at The Hive - including two tries in the final 10 minutes - to help the division's bottom side record a famous 40-36 victory.
Rhinos had one eye on next weekend's Challenge Cup final, but despite adding three tries in the opening 13 minutes the Loiners came undone.
The Bulls have now won their last three games, while the Black and Whites have hit a rough patch with this their eighth winless game in a row.
Peter Aspinall touched down early on for the first of the Claret and Golds' seven tries of the afternoon to put his side on their way to a fourth-straight win.
The result means that there was to be no winning return to league action for Widnes following their Challenge Cup semi-final defeat last time out.
Results in full: Bradford Bulls 34-28 Hull FC, Huddersfield Giants 28-14 Widnes Vikings, London Broncos 40-36 Leeds Rhinos