BACKGROUND: We examined the social cognition determinants of parental supervised toothbrushing guided by the health action process approach (HAPA). METHODS: In a prospective correlational survey study, participants (N = 185, 84.3% women) completed HAPA social cognition constructs at an initial time point (T1), and 12 weeks later (T2) self-reported on their parental supervised toothbrushing behaviour, toothbrushing habit, and action control. RESULTS: Structural equation models exhibited adequate fit with the data when past behaviour and habit were either excluded or included. Intention, self-efficacy, planning, and action control were predictors of parental supervised toothbrushing; intention predicted action planning and coping planning; and self-efficacy and attitude were predictors of intention. Indirect effects of social cognition constructs through intentions, and intentions through planning constructs were also observed. Inclusion of past behaviour and habit attenuated model effects. CONCLUSION: Results indicate that parental supervised toothbrushing is a function of motivational and volitional processes. This knowledge can be used to inform behaviour change interventions targeting parental supervised toothbrushing.