Cyberbullying, i.e., posting malicious comments online, has been identified as a critical social issue in the online and social media context. As a way to prevent cyberbullying, it is important to promote online prosocial behavior. This study examines what motivates people to post benevolent comments as online prosocial behavior in the online context. For this purpose, we first adopt an exploratory study to identify decision factors in terms of social exchange decision making. We then undertake a main study by developing a theoretical research model based on the identified decision factors. The testing results explain what and how those explored factors affect the posting of benevolent comments online in the social media context. The study has its theoretical contribution in demonstrating the decision factors leading to the posting of benevolent comments by extending the social exchange theory. It also has its practical implications by providing guidance for promoting online prosocial behavior.