Background and aims: Smartphones allow users to access social networking sites (SNSs) whenever and wherever they want. Such easy availability and accessibility may increase their vulnerability to addiction. Based on the social cognitive theory (SCT), we examined the impacts of outcome expectancies, self-efficacy, and impulsivity on young Chinese smartphone users' addictive tendencies toward SNSs. Methods: Two hundred seventy-seven Macau young smartphone users (116 males and 161 females; mean age = 26.62) filled out an online Chinese questionnaire concerning their usage of social networking sites via smartphones, addiction tendencies toward SNSs, impulsivity trait, outcome expectancies toward the use, and Internet self-efficacy. Results: The findings revealed that those who spent more time on SNSs also reported higher addictive tendencies. Addictive tendencies were positively correlated with both outcome expectancies and impulsivity, but negatively associated with Internet self-efficacy. These three psychological variables explained 23% of the variance in addictive tendencies. Conclusions: The findings of this study suggest that, compared to demographics, psychological factors provide a better account for addictive tendencies towards SNSs among Chinese smartphone users in Macau. The three psychological risk factors were low Internet self-efficacy, favorable outcome expectancies, and high impulsivity trait. Educational campaigns with screening procedures for high-risk groups are recommended for effective prevention and treatment.