Background: Few studies have measured device-based physical activity and sedentary behaviour following a percutaneous coronary intervention (PCI), with no studies comparing these behaviours between countries using the same methods. The aim of the study was to compare device-based physical activity and sedentary behaviour, using a harmonised approach, following a PCI on-entry into centre-based cardiac rehabilitation in two countries. Methods: A cross-sectional study was conducted at two outpatient cardiac rehabilitation centres in Australia and Sweden. Participants were adults following a PCI and commencing cardiac rehabilitation (Australia n = 50, Sweden n = 133). Prior to discharge from hospital, Australian participants received brief physical activity advice (< 5 mins), while Swedish participants received physical activity counselling for 30 min. A triaxial accelerometer (Actigraph GT3X/ActiSleep) was used to objectively assess physical activity (light (LPA), moderate-to-vigorous (MVPA)) and sedentary behaviour. Outcomes included daily minutes of physical activity and sedentary behaviour, and the proportion and distribution of time spent in each behaviour. Results: There was no difference in age, gender or relationship status between countries. Swedish (S) participants commenced cardiac rehabilitation later than Australian (A) participants (days post-PCI A 16 vs S 22, p < 0.001). Proportionally, Swedish participants were significantly more physically active and less sedentary than Australian participants (LPA A 27% vs S 30%, p < 0.05; MVPA A 5% vs S 7%, p < 0.01; sedentary behaviour A 68% vs S 63%, p < 0.001). When adjusting for wear-time, Australian participants were doing less MVPA minutes (A 42 vs S 64, p < 0.001) and more sedentary behaviour minutes (A 573 vs S 571, p < 0.001) per day. Both Swedish and Australian participants spent a large part of the day sedentary, accumulating 9.5 h per day in sedentary behaviour. Conclusion: Swedish PCI participants when commencing cardiac rehabilitation are more physically active than Australian participants. Potential explanatory factors are differences in post-PCI in-hospital physical activity education between countries and pre-existing physical activity levels. Despite this, sedentary behaviour is high in both countries. Internationally, interventions to address sedentary behaviour are indicated post-PCI, in both the acute setting and cardiac rehabilitation, in addition to traditional physical activity and cardiac rehabilitation recommendations. Trial registrations: Australia: Australian New Zealand Clinical Trials Registry (ANZCTR): ACTRN12615000995572. Registered 22 September 2015, Sweden: World Health Organization Trial Registration Data Set: NCT02895451.