Increased sedentary behaviour (SB) is reportedly associated with mortality and morbidity in cardiovascular disease. However, its relation with physical function is not well understood in phase I cardiac rehabilitation (CR). This study aimed to investigate the rate of SB and the relation between SB and physical function among patients participating in phase I CR. This prospective multicentre cohort study enrolled patients participating in CR from October 2020 to July 2022. Patients with probable dementia and difficulty walking alone were excluded. We used sitting SB time as the index of SB and the Short Performance Physical Battery (SPPB) as the index of physical function at discharge. Patients were divided into the low SB group (< 480 min/day) or high SB group (≥ 480 min/day). We analysed and compared the two groups. The final analysis included 353 patients (mean age: 69.6 years, male: 75.6%), of whom 47.6% (168 of 353) were high SB patients. Total sitting SB time was higher in the high SB group versus the low SB group (733.6 ± 155.3 vs 246.4 ± 127.4 min/day, p < 0.001), and mean SPPB score was lower in the high SB group versus the low SB group (10.5 ± 2.4 vs 11.2 ± 1.6 points, p = 0.001). Multiple regression analysis identified SB as an explanatory variable for total SPPB score (p = 0.017). Patients with high SB had significantly lower SPPB scores than those with low SB. These findings underscore the importance of considering SB when improving physical function. Effective strategies to improve physical function can be developed that consider SB in phase I CR.