In line with a Dynamic Self-Regulatory Processing Model of narcissism (Morf & Rhodewalt, 2001), the present study adopted a motivated self-construction perspective to examine longitudinal associations from adolescent narcissism to youth's social media disclosures, problematic social media use, and smartphone stress, respectively. Adolescents' attention-seeking motives were examined as a mediator of these over-time associations. In line with this model's account of self-image failure, we also expected that narcissistic youth's attention-seeking should increase following experiences of ego threat, such as social rejection. These hypotheses were tested with two waves of self-report data, spaced one year apart, among 307 adolescents aged 12–15 at T1 (Mage = 12.87, SD = 0.75). In line with predictions, earlier adolescent narcissism predicted later social media disclosure, problematic use, and smartphone stress, via increased attention-seeking. Furthermore, a significant interaction between narcissism and perceived social rejection at T1 predicted adolescents' outcomes at T2, via attention-seeking; Participants with a combination of higher narcissism and higher rejection at T1 reported higher levels of attention-seeking at T2. These longitudinal results suggest that narcissistic adolescents' attention-seeking on social media, particularly as a way to recover from social rejection, might backfire and ultimately contribute to an ongoing pattern of self-defeating behavior.