The Jersey Flegg Cup is an under-21 competition administered by the New South Wales Rugby League.
First run in 1961, the competition was originally an under-19 competition and ran over 9–12 weeks alongside the other NSWRL junior competitions. With the advent of the NRL in 1998, the competition moved to an under-20, 26 round, full season format.
In 2003, the competition reverted to the 12-week, under-19 format before returning to its full season, under-20 format in 2004.
The competition ended in 2007 as the National Youth Competition (NYC) was introduced in 2008.
In 2018, the Jersey Flegg Cup was reintroduced as the NYC gave way for state based under-20 competitions. In 2021, the competition moved to an under-21 format.