Preschoolers are expected to transition from parallel play, where they engage in similar activities next to peers, to social play, where they engage in direct interactions with peers. We use longitudinal, multiplex social network analysis to examine the transition between observed parallel and social play over a school year in a 3-year-old classroom (N = 25, 45% girls, 48% White) and a 4-year-old classroom (N = 28, 42.86% girls, 60.71% White). In both classrooms, the existence of a parallel play relationship between two children predicted the formation of a social play relationship between the same two children over time but not vice versa. Findings provide support for a unidirectional, sequential transition from parallel to social play with the same peers. (PsycInfo Database Record (c) 2024 APA, all rights reserved).