This article advances a self-socialization perspective demonstrating that children's understanding of "both" gender categories represents an intergroup cognition that is foundational to the development of gender-stereotyped play. Children's (N = 212) gender category knowledge was assessed at 24 months and play was observed at 24 and 36 months. Higher levels of gender category knowledge and, more specifically, passing multiple measures of knowledge of both gender categories at 24 months was related to increases in play over time with gender-stereotyped toys (doll, truck), but not gender-stereotyped forms of play (nurturing, motion). In contrast to the long-standing focus on self-labeling, findings indicate the importance of intergroup cognitions in self-socialization processes and demonstrate the generalizability of these processes to a diverse sample.