Background Maternal mental morbidity and low perinatal health service utilisation in resource-constrained settings contribute substantially to the global burden of poor maternal, newborn, and child health. The community-based Mbereko+Men program in rural Zimbabwe engaged women and men in complementary activities to improve men's support for women and babies, coparents' equitable, informed health decision-making, and ultimately, maternal mental health and care-seeking for maternal and newborn health services. The study aimed to test the effectiveness of the Mbereko+Men program on maternal mental health at 0-6 months after childbirth. Methods We conducted a cluster-randomised controlled pragmatic trial using a two-arm parallel design with four clusters per arm. Data was data collected through cross-sectional surveys before and after the implementation of the intervention or standard care. Rural health facility catchments in Mutasa District, Zimbabwe, were randomised using a true random number sequence. Survey participants were women who had given birth within 0-6 months and their male coparents. The primary outcome was women's mean Edinburgh Postnatal Depression Scale (EPDS) score. Secondary outcomes captured care-seeking, men's supportive behaviours, and gender dynamics in coparent relationships. Masking was not used. All clusters were included in the analysis. The trial was registered with the Australian New Zealand Clinical Trials Registry (ACTRN12620001014943) in October 2020. Results Between April 13 and May 20, 2016, 457 women and 242 men participated in the pre-intervention survey; between October 19 and November 30, 2017, 433 women and 273 men participated in the post-intervention survey. Women's mean EPDS scores declined in both arms. The decline was 34% greater in the intervention arm (adjusted risk ratio = 0.66; 95% confidence interval = 0.48, 0.90, P = 0.008). Improvements in care-seeking, men's support, and coparents' relationships were detected. Conclusions A low-intensity gender-synchronised ...