This paper develops a model of a professional sports league with network externalities by integrating the theory of two-sided markets into a two-stage contest model. In professional team sports, the competition of the clubs functions as a platform that enables sponsors to interact with fans. In these club-mediated interactions, positive network effects operate from the fan market to the sponsor market, while positive or negative network effects operate from the sponsor market to the fan market. We show that the size of these network effects determines the level of competitive balance within the league. If the market potential of the sponsors is small (large), competitive balance increases (decreases) with stronger combined network effects. We further deduce that clubs benefit from stronger combined network effects through higher profits and that network externalities can mitigate the negative effect of revenue sharing on competitive balance. Finally, we derive implications for improving competitive balance by taking advantage of network externalities. For example, our model suggests that an increase in the market potential of sponsors produces a more balanced league.