Background <p>Postpartum women frequently experience declines in physical activity (PA) resulting from lifestyle changes, caregiving demands, and physical recovery, which increases the risk of long-term physical and mental health issues. While behavioral strategies can promote PA, few interventions address the lifestyle-related barriers specific to postpartum women using objective PA measures, and remote, group-based approaches remain underexplored. This study evaluated the efficacy of an 8-week remotely delivered, group-based PA intervention in increasing objectively measured PA, exercise-related self-efficacy, and psychosocial well-being among postpartum women.</p> Methods <p>In this web-based, two-arm randomized controlled trial, 175 postpartum women (2–6&#xa0;months postpartum) in Japan were allocated to either an intervention (<i>n</i> = 89) or waitlist control group (<i>n</i> = 86). The intervention combined weekly instructor-led online group sessions with a structured home-based exercise program, incorporating behavioral strategies grounded in self-determination and social cognitive theories. The primary outcome was daily moderate-to-vigorous PA (MVPA) measured via triaxial accelerometers. The secondary outcomes included daily step counts, health-related quality of life (HRQoL; Short Form-12 Health Survey version 2), sense of coherence (SOC; Sense of Coherence Scale), and exercise self-efficacy (decisional balance for exercise). Analyses used generalized estimating equations adjusting for baseline values and age.</p> Results <p>Retention was 98%–99%, with 94% attending at least four of six classes. Compared with controls, the intervention significantly increased MVPA by 5.97&#xa0;min/day (95% confidence interval [CI]: 1.34, 10.60; <i>p</i> = 0.012) and daily steps by 576 (95% CI: 73, 1079; <i>p</i> = 0.025). SOC increased by 4.14 points (95% CI: 1.70, 6.58; <i>p</i> &lt; 0.001) and exercise self-efficacy increased (balance score difference: 2.74; 95% CI: 0.71, 4.78; <i>p</i> = 0.008), mainly because of reduced perceived barriers. No significant changes in HRQoL were observed.</p> Conclusions <p>This remote, group-based PA intervention, designed to accommodate the lifestyle demands of the postpartum period, effectively increased PA and enhanced psychosocial resources in postpartum women. By fostering self-efficacy, peer support, and accessible home-based participation, this program may support both short- and long-term physical and mental health. These findings highlight the potential of scalable online PA programs to overcome common postpartum barriers.</p> Trial registration <p>University Hospital Medical Information Network Clinical Trials Registry (UMIN-CTR): UMIN000053478, registered 31 January 2024.</p>