Peers, particularly in-school peers, shape adolescent health behaviors. Yet little is known about in-school peers and self-injurious thoughts and behaviors. This systematic review examined studies of sociometric school-based adolescent peer-friendship networks and associations with self-injurious thoughts and behaviors. A search across five databases identified fifteen eligible studies. Studies were mainly longitudinal (n=13), from two countries (USA and China), of adolescents aged 11-19 at baseline, mostly balanced in gender (46%-56% girls), and from middle/high schools ranging in size (n=348?13482). Studies assessed 1) network structure and 2) exposure to friends' self-injury and suicidality. Friends' self-injurious thoughts and behaviors were associated with adolescents' own similar behaviors, but there was limited evidence for socialization and selection. Sociality and network density were negatively associated with self-injurious thoughts and behaviors, whereas isolation and intransitivity were positively associated. While study heterogeneity made it difficult to draw further conclusions about specific network metrics (e.g., centrality, reciprocity), studies indicate overall that peers matter for these behaviors across adolescent development (e.g., early to late adolescence). Like other adolescent behaviors, the structure of how youth are connected to peers also relates to self-injurious thoughts and behaviors. Future work should examine these processes over the course of adolescent development.