INTRODUCTION: Community mobilization (CM) is increasingly recognized as critical to generating changes in social norms and behaviours needed to achieve reductions in HIV. We conducted a CM intervention to modify negative gender norms, particularly among men, in order to reduce associated HIV risk. METHODS: Twenty two villages in the Agincourt Health and Socio-Demographic Surveillance Site in rural Mpumalanga, South Africa were randomized to either a theory-based, gender transformative, CM intervention or no intervention. Two cross-sectional, population-based surveys were conducted in 2012 (pre-intervention, n = 600 women; n = 581 men) and 2014 (post-intervention, n = 600 women; n = 575 men) among adults ages 18 to 35 years. We used an intent-to-treat (ITT) approach using survey regression cluster-adjusted standard errors to determine the intervention effect by trial arm on gender norms, measured using the Gender Equitable Mens Scale (GEMS), and secondary behavioural outcomes. RESULTS: Among men, there was a significant 2.7 point increase (Beta Coefficient 95% CI: 0.62, 4.78, p = 0.01) in GEMS between those in intervention compared to control communities. We did not observe a significant difference in GEMS scores for women by trial arm. Among men and women in intervention communities, we did not observe significant differences in perpetration of intimate partner violence (IPV), condom use at last sex or hazardous drinking compared to control communities. The number of sex partners in the past 12 months (AOR 0.29, 95% CI 0.11 to 0.77) were significantly lower in women in intervention communities compared to control communities and IPV victimization was lower among women in intervention communities, but the reduction was not statistically significant (AOR 0.53, 95% CI 0.24 to 1.16). CONCLUSION: Community mobilization can reduce negative gender norms among men and has the potential to create environments that are more supportive of preventing IPV and reducing HIV risk behaviour. Nevertheless, we did not observe that changes in attitudes towards gender norms resulted in desired changes in risk behaviours suggesting that more time may be necessary to change behaviour or that the intervention may need to address behaviours more directly. CLINICAL TRIALS NUMBER: ClinicalTrials.gov NCT02129530.