Background HIV and violence prevention programs increasingly seek to transform gender norms among participants, yet how to do so at the community level, and subsequent pathways to behavior change, remain poorly understood. We assessed shifts in endorsement of equitable gender norms, and intimate partner violence (IPV), during the three-year community-based trial of Tsima, an HIV 'treatment as prevention' intervention in rural South Africa. Methods Cross-sectional household surveys were conducted with men and women ages 18-49 years, in 8 intervention and 7 control communities, at 2014-baseline (n = 1,149) and 2018endline (n = 1,189). Endorsement of equitable gender norms was measured by the GEM Scale. Intent-to-treat analyses assessed intervention effects and change over time. Qualitative research with 59 community members and 38 staff examined the change process. Results Nearly two-thirds of men and half of women in intervention communities had heard of the intervention/seen the logo; half of these had attended a two-day workshop. Regression analyses showed a 15% improvement in GEM Scale score over time, irrespective of the intervention, among men (p<0.001) and women (p<0.001). Younger women (ages 18-29) had a decreased odds of reporting IPV in intervention vs. control communities (aOR 0.53;