(Workplace) humour can have positive effects on interpersonal (workplace) relationships. However, sexist humour–a form of subtle discrimination that communicates disparagement or degradation at the expense of (mainly) women in a humorous manner–might be highly detrimental in various ways. Despite sexist humour being pervasive in the workplace, little is known about when, how, and why sexist humour impacts the recipients and their work relationship with the initiator. With two pre-registered studies, theoretically based on affective events theory and social identity theory, this research advances knowledge on the interpersonal effects and gender dynamics of sexist humour at work. In the experimental Study 1, 255 participants rated an actor telling a (non-)sexist joke. In the field Study 2, 170 participants recalled a (non-)sexist humour event they encountered at work. Both studies produced converging results: Recipients of sexist humour were less willing to collaborate with the humour initiator and perceived them as less competent; this was psychologically explained by elevated negative affect. These detrimental effects particularly emerged when a man (vs. woman) initiated sexist humour toward a woman (vs. man).