Purpose: The purpose of this study was to compare the medical, demographic and social cognitive correlates of physical activity (PA) in breast (BCS), prostate (PCS) and colorectal (CRCS) cancer survivors. Methods: A stratified random sample of 2062 BC, PC and CRC survivors diagnosed between 2003 and 2011 was identified by the Nova Scotia Cancer Registry (NSCR) and mailed a questionnaire assessing PA, social-cognitive constructs from the theory of planned behaviour (TPB), and demographic and medical variables. Structural equation modelling was used to conduct path analyses of the TPB within each cancer survivor group and an invariance analysis was used to compare the TPB across groups. Results: A total of 741 completed surveys were analysed. Overall, 42 % of cancer survivors were meeting PA guidelines with no differences among the cancer sites. Treatment-related variables were strong correlates of PA in PC survivors but not for BC or CRC. Body mass index was strongly associated with PA in BC survivors but not PC or CRC. Path analyses within each cancer survivor group showed that intention was significantly associated with PA for CRCS only; planning was significantly associated with PA for BCS and PCS only; and perceived behavioural control (PBC) was significantly associated with PA for PCS only. For intention, PBC and instrumental attitude (IA) were significant correlates in all three cancer survivor groups whereas affective attitude (AA) was significant for BCS and CRCS only; and descriptive norm (DN) was significant for PCS and CRCS only. Invariance analyses revealed significantly stronger relationships for (a) intention to planning for BCS compared to PCS, (b) affective attitude to intention for CRCS compared to PCS, and (c) planning to PA for PCS compared to CRCS. Conclusions: Although BC, PC, and CRC survivors have similar levels of PA, the correlates of their PA may differ. These findings may inform cancer site-specific interventions to promote PA in cancer survivors. © 2013 Springer-Verlag.