Understanding factors that predict toxic and prosocial behavior in massively multiplayer online (MMO) games has drawn a great deal of scholarly attention. Prior work on this topic has primarily focused on individual and technological factors while overlooking the role of interpersonal dynamics. To fill this gap, this study examines if and how players' perceptions of mutual dependence and power in MMO games are related to toxicity and prosocial behavior in games. Results from a survey of 782 Chinese game players suggest that players' perceived power is positively related to prosocial behavior in games. Perceived mutual dependence and power are also indirectly related to prosocial and toxic behaviors through players' passion for games. This study has theoretical implications for scholarship on toxicity, prosocial behaviors, and gameplay, while also providing design and policy implications for MMO games. Lay Summary In this article, we seek to understand why people engage in prosocial and toxic behaviors in massively multiplayer online (MMO) games. We assumed that players' perceptions of mutual dependence among teammates and power as compared to teammates were related to their toxic and prosocial behaviors in MMO games. We also anticipated that players' passion for games was related to their behaviors in games. Through a survey of 782 Chinese game players, we found that when players perceived that they had more power than their teammates, they were more likely to engage in prosocial behaviors. Players' perceptions of mutual dependence and power were positively associated with their passion for games, which was further linked to their prosocial and toxic behaviors in games. Specifically, when players had an uncontrollable urge to play games (i.e., obsessive passion), they were more likely to perform toxicity; when players' passion indicated concordance between games and their pre-existing values (i.e., harmonious passion), players were more likely to carry out prosocial behaviors. We discuss our findings in terms of their theoretical and practical implications.