a b s t r a c tPrevious research has found that some people use Facebook for antisocial purposes, such as for "trolling" or attention-seeking. Conversely, others use Facebook in prosocial, relationship-enhancing ways, such as to increase belonging or to connect with friends. Few studies, however, have investigated differences between men and women in their antisocial and prosocial use of Facebook. The present study sought to address this research gap by examining whether these sex differences might be explained by narcissism and relational self-construal (i.e., the extent to which an individual defines their self in terms of close relationships). To this end, 573 participants living in the United States completed measures of narcissism, relational self-construal, and motives for using Facebook. Results revealed that men reported more antisocial motives for using Facebook than did women, which was explained by their greater narcissism. Conversely, women reported stronger prosocial motives for using Facebook, which was explained by their more relational self-construal. We discuss ways that these findings can contribute to the development of interventions to promote prosocial online behaviour.