Objective: Interpersonal relationships are important predictors of health outcomes, and interpersonal influences on behaviours may represent key mechanisms underlying such effects. Most health behaviour theories focus on intrapersonal factors and may not adequately account for interpersonal influences. We evaluate a dyadic extension of the Theory of Planned Behaviour by examining how adolescent and parent characteristics (attitudes, subjective norms, perceived behavioural control, intentions) predict not only their own, but each other’s intentions/behaviours. Design: Using the Actor-Partner Interdependence Model, we analyse responses from 1,717 parent-adolescent dyads from the Family Life, Activity, Sun, Health, and Eating (FLASHE) study. Main Outcome Measures: Adolescents/parents completed self-reports of their fruit and vegetable consumption, junk food and sugary drinks consumption, engagement in physical activity, and engagement in screen time sedentary behaviours. Results: We find that parent and adolescent characteristics predict each other’s health-relevant intentions/behaviours, and that these effects emerge above the predictive ability of individuals’ own characteristics. Parent/adolescent characteristics predict each other’s outcomes at similar levels, but parent characteristics more strongly predict adolescent intentions, whereas adolescent characteristics more strongly predict parent behaviours. Conclusions: Parents and adolescents may bidirectionally influence each other’s health intentions/behaviours. This highlights the importance of dyadic models of health behaviour and suggests intervention targets.