Sunshine Circles is a teacher-led group process using social-relationship principles from Theraplay®. This study, conducted across 6 preschool sites in the midwestern United States, was the first to examine empirical outcomes against a control group for this program. Students in these teacher-led, play-based groups improved significantly compared with controls in social-emotional skills, behavioral regulation, problem-solving, and fine motor control. Specific improvements occurred in domains of managing feelings, cooperation, accepting limits, peer interactions and friendships, and solving social problems. Furthermore, structured teacher observation measurements yielded data indicating improvement in teacher classroom performance. Interviews with teachers confirmed that the intervention subjectively increased classroom cohesion, improved teacher–student relationships, and improved overall classroom behavior. These findings have implications for both classroom best practice and teacher education.