Narcissistic youth use social media to engage in a variety of self-promotional behaviors, which have either antisocial or prosocial characteristics. Differing views exist to explain the processes underlying narcissistic self-promotion, either characterizing these actions as intentional, or as impulsive. This study compared intentional attention-seeking and impulsivity as potential mediators of relations between narcissism and both aggressive (i.e., cyberbullying) and prosocial online behavior, and examined whether youth's loneliness might strengthen these associations. Among Chinese early adolescents (N = 213, M[subscript age] = 13.26), narcissism positively predicted youth-reported cyberbullying offending and online prosocial behavior. Loneliness moderated the link between narcissism and attention-seeking, but not impulsivity. Among adolescents higher in loneliness, narcissism indirectly predicted cyberbullying and online prosocial behaviors via attention-seeking. These results highlight narcissism, loneliness, and their interplay as potential predictors of youth's social media behaviors. Links with attention-seeking, in particular, suggest that educators and practitioners might target youth's conscious expectations for social rewards when counseling narcissistic adolescents about self-promotional social media use.