Adolescents’ close friendships are an important and unique learning context in which adolescents can practice and hone their emotion regulation skills within an egalitarian, supportive relationship structure that provides important feedback on the effectiveness of the regulation strategies. This longitudinal study examined whether adolescents’ involvement in supportive reciprocal friendships influenced the way in which they regulated angry feelings arising in these friendships. A sample of 299 German adolescents began a 30-month, 3-wave longitudinal study in grade 7 (151 boys, M age = 12.6 years; 100% White). They completed a social network inventory (LueNIC), a peer-nomination measure, and the questionnaire on Strategies of Anger Regulation for Adolescents (SAR-A) in every wave. Cross-lagged-panel modeling indicated a pattern of socialization effects even when controlling for previous friendship involvement, previous anger regulation, peer acceptance, gender, classroom membership, and possible friendship selection influences. Adolescents with more reciprocal friends at Time 1 (T1) reported using aggressive strategies of anger regulation (i.e., verbal and relational aggression, fantasies of revenge) and ignoring the friend less often at Time 2 (T2). Similar results were obtained between T2 and Time 3 (T3). There was a marginally significant effect for one of three non-aggressive strategies such that a higher involvement in friendships at T2 explained more reappraisal of the anger-eliciting event at T3 but significant effects did not emerge for the strategies of redirection of attention and explanation and reconciliation. The results are discussed within a socialization of emotion framework with implications for social skills training modules.