To investigate the utility of the protection motivation theory (PMT) for explaining physical activity (PA) in an adult population with type 1 diabetes (T1D) and type 2 diabetes (T2D).Cross-sectional and 6-month longitudinal analysis using PMT.Two thousand three hundred and eleven individuals with T1D (N=697) and T2D (N=1,614) completed self-report PMT constructs of vulnerability, severity, response efficacy, self-efficacy, and intention, and PA behaviour at baseline and 6-month follow-up. Multi-group structural equation modelling was conducted to: (1) test the fit of the PMT structure; (2) determine the similarities and differences in the PMT structure between the two types of diabetes; and (3) examine the explained variance and compare the strength of association of the PMT constructs in predicting PA intention and behaviour.The findings provide evidence for the utility of the PMT in both diabetes samples (chi(2)/df=1.27-4.08, RMSEA=.02-.05). Self-efficacy was a stronger predictor of intention (beta=0.64-0.68) than response efficacy (beta=0.14-0.16) in individuals with T1D or T2D. Severity was significantly related to intention (beta=0.06) in T2D individuals only, whereas vulnerability was not significantly related to intention or PA behaviour. Self-efficacy (beta's=0.20-0.28) and intention (beta's=0.12-0.30) were significantly associated with PA behaviour.Promotion of PA behaviour should primarily target self-efficacy to form intentions and to change behaviour. In addition, for individuals with T2D, severity information should be incorporated into PA intervention materials in this population.