Purpose: Seasons and weather conditions might influence participation in physical activity and contribute to differences between countries. This study aimed at investigating whether there were differences in physical activity levels between Norwegian, Danish and Australian people with chronic obstructive pulmonary disease (COPD), and establishing if any variations in physical activity were attributable to seasons. Patients and methods: A cross-sectional study where study subjects were people with COPD who participated in two separate clinical trials: the iTrain study (Norway, Denmark, and Australia) and the HomeBase study (Australia). Physical activity was objectively assessed with an activity monitor; variables were total energy expenditure, number of daily steps, awake sedentary time, light, and moderate-to-vigorous intensity physical activity. Differences in physical activity between countries and seasons were compared, with adjustment for disease severity. Results: In total, 168 participants were included from Norway (N=38), Denmark (N=36) and Australia (N=94). After controlling for disease severity, time spent in awake sedentary time was greater in Danish participants compared to the other countries (median 784 minutes/day [660-952] vs 775 minutes/day [626-877] for Norwegians vs 703 minutes/day [613-802] for Australians, P=0.013), whilst time spent in moderate to vigorous physical activity was lower (median 21 minutes/day [4-73] vs 30 minutes/day [7-93] for Norwegians vs 48 minutes/day [19-98] for Australians, P=0.024). Participants walked more during summer (median 3502 [1253-5407] steps/day) than in spring (median 2698 [1613-5207] steps/day), winter (median 2373 [1145-4206] steps/day) and autumn (median 1603 [738-4040] steps/day), regardless of geography. The median difference between summer and other seasons exceeded the minimal clinically important difference of 600 steps/day. However, the differences were not statistically significant (P=0.101). Conclusion: After controlling for disease severity, Danish participants spent more time in an awake sedentary state and less time in moderate to vigorous physical activity than their counterparts in Norway and Australia. People with COPD increased their physical activity in summer compared to other seasons. Weather conditions and seasonal variations may influence outcomes in clinical trials and health registries measuring physical activity over time, irrespective of the interventions delivered, and should be taken into account when interpreting results.