While more research is emerging about the development of masculinity during adolescence, not much is known about how masculine-type behaviors develop over time in middle to late adolescence within the context of friendships and peer experiences. This study examined trajectories of masculine-typed behavior from ages 14 to 17. Multilevel modeling was used to account for cross-time and within-time variability in masculine-typed behavior and examined the role of positive and negative peer experiences in predicting this variability. This was done in a sample of 334 U.S. adolescents (51% boys; 50% White, 19% Black, 15% Latina/o/e). At the between-person level, boys and girls decreased in masculine-typed behavior over time. At the within-person level, negative peer experiences predicted fluctuations toward greater masculine-typed behavior, whereas friend support predicted fluctuations toward less masculine-typed behavior. Adolescence is a key period for navigating masculinity norms, and peer experiences are a key context for the development of masculine-typed behavior.