This study aims to understand the relationship between narcissistic tendencies in adolescents and their SNS behaviors (posting, sharing, reassurance-seeking, and scrolling newsfeed), with a particular interest in the possible mediating role of adolescent interpersonal and social-avoidance motives for SNS use. To do this, a survey was conducted on 303 students from three different middle schools in Seoul. Correlation analysis and mediation analysis using SPSS Process Macro were performed on the data collected. The results were as follows. Narcissistic tendencies in adolescents were found to be positively correlated with interpersonal and social-avoidance motives, and posting, sharing, and reassurance-seeking behaviors while being negatively correlated with scrolling behaviors. Likewise, the two motives for SNS use were found to be positively correlated with posting, sharing, and reassurance-seeking behaviors and negatively correlated with scrolling behaviors. Next, mediation analyses were conducted with gender and weekly amount of SNS screen time as controlling variables. For posting and reassurance seeking behaviors, a partial mediation of both interpersonal and social-avoidance motives was found while a full mediation of both motives was found for sharing behaviors. On the other hand, for scrolling, narcissistic tendencies were only partially mediated by the interpersonal motive, and not at all by the social- avoidance motive. This study verifies positive effects of narcissistic tendencies on posting, sharing, and reassurance-seeking SNS behaviors and a negative one on scrolling behaviors and further supports the hypothesis for the mediating role of interpersonal and/or social-avoidance motives for SNS use in these effects.