This study was conducted to provide a comprehensive understanding of the formation of social media users’ rumor combating behavior during crises by merging the theory of planned behavior and the norm activation model into one theoretical framework. Based on a sample of 394 social media users, the descriptive statistical analysis showed two different kinds of gaps. “Higher awareness but lower behavior gap” represented gaps from higher-scoring constructs (including awareness of adverse consequences caused by rumors, ascribed responsibility of adverse consequences, sense of obligation and behavioral intention to combat rumors) to lower-scoring actual behavior, whereas “lower attitude but higher behavior gap” represented the gap from lower-scoring attitudes toward rumor combating to higher-scoring actual behavior. The results of the structural analysis demonstrated that the proposed model had a satisfactory level of prediction power for pro-social rumor combating behavior. The findings also supported all the hypothesized relationships among the model constructs except the direct influences of subjective norms and perceived behavioral control on behavioral intention and verified the mediating roles of the model constructs. In particular, subjective norms, perceived behavioral control and awareness of adverse consequences were the three most important predictors of actual rumor combating behavior.