PURPOSE: This study explored perceived changes in health behaviours and body weight following a cancer diagnosis and investigated related sociodemographic and clinical characteristics. METHODS: Individuals living with and beyond breast, prostate, or colorectal cancer (N = 5835) completed the 'Health and Lifestyle After Cancer Survey' which explored their perceptions of health behaviour change following a cancer diagnosis. Multinomial logistic regressions were conducted with perceived changes as dependent variables, and sociodemographic and clinical characteristics as independent variables. RESULTS: Approximately half of the participants did not perceive changes in their physical activity, alcohol intake and body weight, and most did not perceive dietary changes. Less than a third of individuals perceived positive health behaviour changes (11.7% increased physical activity, 24.3% healthier diet, and 31.3% decreased alcohol intake), 35.9% perceived decreases in physical activity, and 27.0% perceived increases in body weight, whereas 19.2% perceived decreases in body weight. Individuals with no education, who were unmarried, and with anxiety/depression and pain/discomfort, were more likely to perceive changes in physical activity, body weight, and diet, but in different directions. Participants of younger age were more likely to perceive increases in their physical activity, a healthier diet, and increases in body weight. CONCLUSION: Following a diagnosis of cancer, a large proportion of individuals perceived that their health behaviours were unchanged. However, some groups of individuals were more likely to perceive positive changes, whereas others were more likely to perceive negative changes, with differences also observed according to the type of health behaviour. IMPLICATIONS FOR CANCER SURVIVORS: Participants with no education, who were unmarried, with anxiety/depression and pain/discomfort, may be more at risk of experiencing negative health behaviour changes post-diagnosis. Clinicians should consider targeting health behaviour support to prevent worse outcomes in the long term.