Food waste is a critical environmental issue, with household food waste representing the largest contributor to overall food loss. While previous research has linked food waste behaviour to psychosocial factors, the influence of dietary preference on food waste remains underexplored. This study applied the Theory of Planned Behaviour (TPB) to examine whether diet type (vegan, vegetarian, or omnivore) predicts household food waste behaviour. A total of 363 participants completed an online survey measuring food-related routines, household skills, TPB constructs, and pro-environmental attitudes. Results indicated that household food waste was significantly predicted by perceived behavioural control, shopping and planning routines, and household skills. Pro-environmental attitudes were weakly associated with reduced food waste but significantly predicted intentions not to waste food. Vegans reported significantly stronger pro-environmental attitudes and less food waste than omnivores, with vegetarians falling in between. These findings support an extended TPB model and highlight the relevance of dietary identity in shaping food waste behaviour. Targeting food-related routines and enhancing perceived control may improve the effectiveness of food waste reduction campaigns, particularly when tailored to dietary groups.