The Jersey Flegg Cup was a junior rugby league competition played in New South Wales, contested among teams made up of players aged under 20. The competition was administered by the New South Wales Rugby League (NSWRL) and run concurrently with the National Rugby League (NRL). It ceased at the end of the 2007 season to make way for the NRL's new Under 20s tournament, the National Youth Competition (referred to as the Toyota Cup), which commenced in 2008.

The Cup began as an Under 19s age group competition, changing to Under 20s with the advent of the NRL in 1998. It was originally held as part of the NSWRL's Junior Representative Competition, which was played over 9–12 weeks early in the season. It then evolved to become a full season competition played over 26 rounds, culminating with the Grand Final held on the same day as the First Grade Premiership decider.

The competition was named for Harry "Jersey" Flegg, a former player (playing in 1908 and 1909 for Eastern Suburbs) and prominent administrator of the game between 1909 and 1960.