Grievances about being a man have surged in recent years, with men's rights activists claiming that men are unjustly disadvantaged relative to women. Group-based relative deprivation may explain such perceptions. Across one correlational study among White American men (N = 195) and one experimental study among Black and White American men (N = 492), we examined whether low trait modesty may explain why some men experience group-based relative deprivation, and endorse normative and non-normative men's rights activism. Across both studies, there were significant negative direct and indirect effects of trait modesty on men's rights activism. Specifically, trait modesty negatively predicted endorsement of normative and non-normative men's rights activism, and these effects were mediated by lower group-based relative deprivation. These findings advance our understanding of how personality traits shape perceptions of group-based disadvantage and collective action in defence of men's rights.