Individuals often need to self-promote for social and professional recognition. In this paper, we investigate the existence of a gender gap in self-promotion of a prosocial action and explore its link with modesty norms. Using a novel experiment that combines both lab and field elements, we show that women are up to five times less likely to self-promote than men. We find suggestive evidence that the difference in behaviour across gender is determined by women’s social image concerns of being perceived as immodest. We find that the provision of a justification to self-promote has two important consequences: (i) it leads to an increase in self-promotion by women and (ii) contributes to the elimination of the gender gap in self-promotion behaviour.