Executive functions (EFs) are foundational for the development of cognitive and social capacities critical for school readiness. The importance of promoting EF skills prior to school entry is increasing recognized but few preschool classroom-based interventions specifically target EF skills. The aim of this study was to evaluate the effectiveness of a classroom-based intervention, Red Light Purple Light (RLPL), to enhance 4-year-old children's EF skills. Fifteen early childhood education (ECE) centers in Auckland, New Zealand participated. There was a total of 212 consented 4-year-old children across the 15 centers. Centers were randomized to either the RLPL intervention (twice weekly circle time games sessions over 8 weeks) or a waitlist control group (the usual curriculum). Children's EF skills were measured at pre-, post-, and 4-month postintervention using the Head-Toes-Knees-Shoulder (HTKS) and Dimensional Change Card Sort (DCCS) tasks. Teacher ratings of behavioral self-regulation were collected with the Child Behavior Rating Scale (CBRS). Hierarchical linear mixed models were used to estimate the intervention effects, taking into account baseline scores, English vocabulary, age, and gender. Post-intervention interviews were conducted with 15 teachers. Significant short-term intervention effects were found for increases in EFs, based on DCCS scores. At 4-month follow-up, there were greater gains in CBRS scores for intervention group children. Teacher interviews indicated additional benefits of participation for children in self-control, language, and social skills. The study provides support for the effectiveness of the RLPL intervention, delivered by teachers in preschool classrooms, for improving children's EF skills.