The gendered nature of children’s peer relationships has received little attention as a mechanism of change for students’ social-emotional competencies. To address this gap, we tested the effects of an easy-to-implement universal intervention (Buddy Up) that paired preschoolers with other-gender peers for enjoyable, cooperative, and structured classroom activities. We considered whether the Buddy Up intervention, relative to a control condition, predicted changes in children’s aggression and prosocial behavior. Participants were 140 preschoolers (53.6% boys; Mage = 56.49 months; 78% Mexican/Mexican-American) from the Southwestern United States. Following a 2-week pretest period in January (T1), teachers in intervention classrooms implemented the Buddy Up program, which continued until the end of the school year. Post-test data were collected in May (T2). As hypothesized, Buddy Up was associated with increased prosocial behavior and reduced aggression. (author abstract)