This paper develops and expands upon social identity theory as an explanation for gambling among youth engaged in team sport. Analysing longitudinal data for over 4500 20-year-olds from the Growing Up in Ireland study, reveals that online gambling increased from 2.6 to 9.3% between 17 and 20 years in the cohort, with the increase driven by males. A statistically significant positive association is uncovered between playing team sports and regularly gambling, as well as online gambling behaviour, independent of socio-demographic and other risk factors for males but not for females. The findings provide support for a dose-response like effect for males, where a longer period of participation in team sports is associated with a higher likelihood of engaging in gambling behaviour compared to shorter periods. Implications of the findings for policy and practice are discussed.