Objective: The purpose of this study was to investigate the relationship between body mass index (BMI) class and physical activity and sedentary behavior in patients with acute coronary syndrome (ACS) during cardiac rehabilitation (CR). Methods: This study was a secondary analysis of the OPTICARE trial. Physical activity and sedentary behavior were measured in participants with ACS (n = 359) using actigraphy at baseline, directly after completion of a multidisciplinary 12-week exercise-based CR program and 9 months thereafter. Outcome measures were step count and duration of time (percentage of wear time) spent in light physical activity, moderate-to-vigorous physical activity, and sedentary behavior. Participants were classified as normal weight (BMI = 18.5-24.99 kg/m2; n = 82), overweight (BMI = 25.0-29.99 kg/m2; n = 182), or obese (BMI ≥ 30.0 kg/m2; n = 95). Linear mixed-effects models were applied to study the relationship between BMI class and physical activity and sedentary behavior. Results: At the start of CR, compared with participants with normal weight, participants with obesity made on average 1.11 steps fewer per minute (952 steps/d), spent 2.9% (25 min/d) less time in light physical activity, and spent 3.31% (28 min/d) more time in sedentary behavior. Participants of all BMI classes improved their physical activity and sedentary behavior levels similarly during CR, and these improvements were maintained after completion of CR. Conclusion: Participants with ACS who had obesity started CR with a less favorable physical activity and sedentary behavior profile than that of participants with normal weight. Because all BMI classes showed similar improvement during CR, this deficit was preserved. Impact: This study indicates that reconsideration of the CR program in the Netherlands for patients with ACS and obesity is warranted, and development of more inclusive interventions for specific populations is needed. A new program for people with obesity should include added counseling on increasing physical activity and preventing sedentary behavior to facilitate weight loss and reduce mortality risk. Lay Summary: People with ACS who have obesity are less active and sit more than individuals with normal weight, both during and after CR. This study suggests that CR needs to be changed to help individuals increase their physical activity to help them lose weight and reduce their risk of death.