Despite striking gender differences in men's and women's engagement in past prevention programmes to stop violence against women (VAW), few empirical studies have determined why such gender differences arise. We did a grounded theory study of a large-scale NGO programme in informal settlements in Mumbai, India, aiming to analyse how gender affects participation in community action to address VAW. We did 27 focus group discussions and 31 semi-structured interviews with 77 women and 36 men, as well as with 9 NGO staff. We supplemented qualitative data with quantitative monitoring data on referrals to NGO counselling centres. We found that male participants in the NGO programme not only reported violence to the NGO at lower rates but took less intensive action to support survivors. When they did engage, they more often defended perpetrators or asked survivors to accommodate them than female participants. These differences could be explained by a greater (1) affective response to VAW (2) perceived stake in addressing VAW (3) sense of empowerment from taking action, and (4) perceived NGO support for self among women compared to men. Differences were evident even between male and female participants who had taken part in NGO activities for years. We theorise that these differences ultimately stem from men and women's structurally different position in a gender unequal society. Our findings identify key motivational barriers to address to improve the effectiveness of programmes to prevent VAW. Given the greater barriers to men's community action compared to women's community action, it may be useful to recognize the value of prevention programmes with differing levels of male engagement, including women-only programmes and programmes in which male community participants play a secondary role and are not expected to be as active as female participants.