<p>Subjective well-being (SWB) is an important indicator of positive adolescent development and is closely related to social relationships. However, less is known about which friendship characteristics are most relevant to adolescent well-being and through which relational processes these associations may unfold over time. The present study addressed these questions using a three-wave time-lagged design with 1,085 ninth-grade students from five middle schools in China. At Time 1, friendship characteristics, including the number of close friends, the number of general friends, contact frequency, and friendship heterogeneity, were assessed. At Time 2, interpersonal trust and perceived social support were measured. At Time 3, SWB was operationalized as a composite index comprising positive affect, life satisfaction, and reverse-scored negative affect (SWB = positive affect + life satisfaction − negative affect). Correlational analyses, exploratory network analysis, and time-lagged mediation analyses were conducted. The results indicated that the number of close friends, rather than other friendship characteristics, showed the most consistent associations with adolescents’ later SWB. Perceived social support significantly mediated the association between the number of close friends and subsequent SWB. In addition, interpersonal trust and perceived social support formed a significant sequential mediating pathway, whereas interpersonal trust alone did not function as an independent mediator. These findings highlight the importance of emotionally meaningful friendships in adolescent well-being and suggest that their associations with later well-being are linked primarily to supportive relational processes. The study contributes to a more differentiated understanding of how friendship characteristics are associated with adolescent SWB in a three-wave time-lagged framework.</p>