Background: This study examined the effectiveness of a 12-month activity tracker-based intervention on activity behavior among recent retirees (Enhancing physical ACTivity and healthy aging among recent REtirees [REACT]) in subgroups based on body mass index. Methods: REACT trial randomized 231 participants (mean age 65.2) into intervention and control groups. Main outcomes were accelerometer-measured moderate-to-vigorous (MVPA) and light physical activity (LPA) and sedentary time (SED) measured at baseline and 3-, 6-, and 12-month follow-ups. As a post hoc analysis, the intervention effect was examined among participants with normal weight (n = 77), overweight (n = 89), and obesity (n = 61). Results: An intervention effect was observed among participants with obesity in LPA (time × group P = .045) mirrored by a similar, albeit nonsignificant, effect in SED (P = .067), but not in MVPA (P = .92). A transient increase of 41 minutes per day (95% confidence interval, 14 to 68) in LPA was observed at 6 months among the intervention group, with a concomitant decrease of 42 minutes per day (−72 to −12) in SED. However, these changes were not maintained at 12 months. No between-group differences in changes over time were observed among participants with normal or overweight. Conclusions: Activity trackers may be particularly suitable for promoting changes in LPA and SED among older adults with obesity. However, their long-term effectiveness might be limited.