To reach the SDG 12.3 target, understanding food waste behaviour (FWB) is fundamental, especially among young people. As future generations of parents, leaders, and citizens, they might potentially solve food sustainability issues and other matters arising from food waste. In such a context, this study extends the theory of planned behaviour (TPB) by including personal norms and environmental concern to explain and to predict the FWB of young Italians who belong to Generation Y and Generation Z. The theoretical model was empirically tested by administering a questionnaire to 322 Italian young people and by applying two distinct PLS-SEM models for Generation Y and Generation Z. Our results support not only the main relationship suggested by the TPB model with respect to FWB but also that, in affecting it, personal norms and environmental concern play different roles according to the generations. Understanding the mechanisms through which personal norms and environmental concern translate into reducing food waste is crucial to defining interventions aimed at changing youth behaviour. It could represent a relevant step towards preventing food waste and promoting sustainable management of food chains.