Drawing from social exchange theory, this research develops a mediated-moderation model that examines the direct and indirect effects of participation in fun activities on three knowledge management behaviors (i.e., knowledge sharing, knowledge hiding, knowledge manipulating) and investigates the mediating role of positive affect and the moderating role of citizenship pressure on these relationships. A three-wave, two-source sample (n = 163) of employees belonging to a high-tech start-up in Canada is used to test this model. Results highlight the importance of positive affect by showing the effects of participation in fun activities on knowledge management behaviors is dependent on whether or not participation in fun activities leads to positive affect. Data also shows citizenship pressure moderates the direct relationship between participation in fun activities and knowledge manipulating, as well as the indirect relationship between participation in fun activities and both knowledge sharing and hiding. These results highlight the theoretical and practical importance of both positive affect and citizenship pressure in understanding the dynamic relationship between workplace fun and knowledge management.