OBJECTIVE: More than 200 million children globally do not attain their developmental potential. We hypothesized that a parent training program could be integrated into primary health center visits and benefit child development. METHODS: We conducted a cluster randomized trial in the Caribbean (Jamaica, Antigua, and St Lucia). Fifteen centers were randomly assigned to the control (n = 250 mother-child pairs) and 14 to the intervention (n = 251 mother-child pairs) groups. Participants were recruited at the 6- to 8-week child health visit. The intervention used group delivery at 5 routine visits from age 3 to 18 months and comprised short films of child development messages, which were shown in the waiting area; discussion and demonstration led by community health workers; and mothers’ practice of activities. Nurses distributed message cards and a few play materials. Primary outcomes were child cognition, language, and hand-eye coordination and secondary outcomes were caregiver knowledge, practices, maternal depression, and child growth, measured after the 18-month visit. RESULTS: Eight-five percent of enrolled children were tested (control = 210, intervention = 216). Loss did not differ by group. Multilevel analyses showed significant benefits for cognitive development (3.09 points; 95% confidence interval: 1.31 to 4.87 points; effect size: 0.3 SDs). There were no other child benefits. There was a significant benefit to parenting knowledge (treatment effect: 1.59; 95% confidence interval: 1.01 to 2.17; effect size: 0.4). CONCLUSIONS: An innovative parenting intervention, requiring no additional clinic staff or mothers’ time, was integrated into health services, with benefits to child cognitive development and parent knowledge. This is a promising strategy that merits further evaluation at scale.