People who express positive emotion usually have better social outcomes than people who do not, and suppressing the expression of emotions can have interpersonal costs. Nevertheless, social convention suggests that there are situations in which people should suppress the expression of positive emotions, such as when trying to appear humble in victory. The present research tested whether there are interpersonal costs to expressing positive emotions when winning. In Experiment 1, inexpressive winners were evaluated more positively and rated as lower in hubristic—but not authentic—pride compared with expressive winners. Experiment 2 confirmed that inexpressive winners were perceived as using expressive suppression to downregulate their positive emotion expression. Experiment 3 replicated the findings of Experiment 1, and also found that people were more interested in forming a friendship with inexpressive winners than expressive winners. The effects were mediated by the perception that the inexpressive winner tried to protect the loser’s feelings. This research is the first to identify social costs of expressing positive emotion, and highlights the importance of understanding the situational context when determining optimal emotion regulation strategies.(PsycINFO Database Record (c) 2016 APA, all rights reserved)