John Joseph O'Neill (born 26 August 1957, Derry, Northern Ireland) is the rhythm guitarist and principal songwriter within the punk rock/new wave band The Undertones. O'Neill, along with his younger brother Vincent and friends Feargal Sharkey, Michael Bradley and Billy Doherty, founded The Undertones in 1975, although Vincent O'Neill was replaced the following year with his younger brother, Damian O'Neill, who became the lead guitarist within the band.

John O'Neill was the principal songwriter in The Undertones: writing the majority of the band's singles and a number of album tracks (occasionally collaborating with his brother Damian or Michael Bradley). Songs written by John O'Neill include the singles "Teenage Kicks", "Jimmy Jimmy", "Here Comes The Summer","You've Got My Number" and notable album tracks such as "When Saturday Comes" and "You're Welcome".