It has been hypothesised that physical activity may be useful as a smoking cessation intervention for young adults. In order to inform such interventions, this study evaluated the theory of planned behaviour (TPB) for understanding physical activity behaviour in young smokers. Regular smokers aged 16-19 years (N=124), self-reported physical activity and all TPB components. Physical activity behaviour was significantly explained by both intention and perceived behavioural control (PBC), with both intention and PBC making significant contributions to the model. Intention was significantly explained by attitude, subjective norm (SN) and PBC, with attitude, SN and PBC all making significant contributions to the model. The TPB may be a useful framework for guiding physical activity interventions among young smokers.