Importance: Behavior problems are one of the most common mental health disorders in childhood and can undermine children's health, education, and employment outcomes into adulthood. There are few effective interventions for early childhood. Objective: To test the clinical effectiveness of a brief parenting intervention, the Video-feedback Intervention to promote Positive Parenting and Sensitive Discipline (VIPP-SD), in reducing behavior problems in children aged 12 to 36 months. Design, Setting, and Participants: The Healthy Start, Happy Start study was a 2-group, parallel-group, researcher-blind, multisite randomized clinical trial conducted via health visiting services in 6 National Health Service trusts in England. Baseline and 5-month follow-up data were collected between July 30, 2015, and April 27, 2018. Of 818 eligible families, 227 declined to participate, and 300 were randomized into the trial. Target participants were caregivers of children who scored in the top 20% for behavior problems on the Strengths and Difficulties Questionnaire. Participants were randomly allocated on a 1:1 basis to receive either VIPP-SD (n = 151) or usual care (n = 149), stratified by site and number of participating caregivers. Analysis was performed on an intention-to-treat basis. Statistical analysis was performed from September 5, 2019, to January 17, 2020. Interventions: All families continued to access usual care. Families allocated to VIPP-SD were offered 6 home-based video-feedback sessions of 1 to 2 hours' duration every 2 weeks. Main Outcomes and Measures: The primary outcome was the score on an early childhood version of the Preschool Parental Account of Children's Symptoms, a semistructured interview of behavior symptoms, at 5 months after randomization. Secondary outcomes included caregiver-reported behavior problems on the Child Behavior Checklist and the Strengths and Difficulties Questionnaire. Results: Among 300 participating children (163 boys [54%]; mean [SD] age, 23.0 [6.7] months), primary outcome data were available for 140 of 151 VIPP-SD participants (93%) and 146 of 149 usual care participants (98%). There was a mean difference in the total Preschool Parental Account of Children's Symptoms score of 2.03 (95% CI, 0.06-4.01; P =.04; Cohen d = 0.20 [95% CI, 0.01-0.40]) between trial groups, with fewer behavior problems in the VIPP-SD group, particularly conduct symptoms (mean difference, 1.61 [95% CI, 0.44-2.78]; P =.007; d = 0.30 [95% CI, 0.08-0.51]). Other child behavior outcomes showed similar evidence favoring VIPP-SD. No treatment or trial-related adverse events were reported. Conclusions and Relevance: This study found that VIPP-SD was effective in reducing symptoms of early behavior problems in young children when delivered in a routine health service context. Trial Registration: isrctn.org Identifier: ISRCTN58327365.