Objective: Sun safety is crucial for preventing skin cancer. This study evaluated a school‐based intervention based on the theory of planned behaviour (TPB), which aimed to encourage sun‐protective behaviour among adolescents. Methods: Secondary school students (N = 382; 61.1% female; Mage = 13.73 y) in Queensland, Australia, participated in the study. Schools were randomly allocated to an intervention or control group. The intervention focussed on fostering positive attitudes, increasing perceptions of normative support, and strengthening control perceptions. Participants completed questionnaires assessing the TPB variables and sun‐protective behaviour (weekday and weekend) 1 week before intervention (time 1), 1 week after intervention (time 2), and 4 weeks after intervention (time 3). Results: With baseline between‐group differences in TPB variables matched, repeated‐measures multivariate analysis of variance was used to evaluate the Time × Condition effects across time. Multigroup comparisons using path models traced the intervariable changes. From times 1 to 3, a significant improvement in weekend sun‐protective behaviour was identified in the intervention group (but not the control group), whereas cognitions showed no significant changes across time for either conditions. Multigroup comparisons on path coefficients between the intervention and control group participants indicated that the intervention group members formed stronger positive associations between perceived behavioural control and intention at time 2 and between perceived control and behaviour at time 3. Conclusion: The significant behavioural change on weekends highlights the value of targeting control perceptions, which may encourage adolescents' sun‐protective behaviour. Further studies are needed to understand the absence of significant changes in weekday sun‐safe behaviour among this at‐risk cohort.