Physical activity (PA) plays a key role in the management of Type 1 (T1D) and Type 2 diabetes (T2D) but there are few theory-based, effective programs to promote PA for individuals with diabetes. The purpose of this study was to investigate the utility of the Theory of Planned Behaviour (TPB) in understanding PA in an adult population with T1D or T2D. A total of 2311 individuals (691 T1D; 1614 T2D) completed self-report TPB constructs of attitude, subjective norms, perceived behavioural control (PBC), intention and PA at baseline and 1717 (524 T1D; 1123 T2D) completed the PA measure again at 6-month follow-up. Multi-group Structural Equation Modelling was conducted to: (1) test the fit of the TPB structure (2) determine the TPB structural invariance between the two types of diabetes and (3) to examine the explained variances in PA and compare the strength of associations of the TPB constructs in the two types of diabetes. The TPB constructs explained ≥40% of the variance in intentions for both diabetes groups. In cross-sectional models, the TPB accounted for 23 and 19% of the variance in PA for T1D and T2D, respectively. In prospective models, the TPB explained 13 and 8% of the variance in PA for T1D and T2D, respectively. When adjusting for past PA behaviour, the impact of PBC and intention on behaviour was reduced in both groups. The findings provide evidence for the utility of the TPB for the design of PA promotion interventions for adults with either T1D or T2D.