IMPORTANCE: Mindfulness curricula can improve physician burnout, but implementation during residency presents challenges. OBJECTIVE: To examine whether a novel mindfulness curriculum implemented in the first 6 months of internship reduces burnout. DESIGN, SETTING, AND PARTICIPANTS: This pragmatic, multicenter, stratified cluster randomized clinical trial of a mindfulness curriculum randomized 340 pediatric interns to the intervention or control arm within program pairs generated based on program size and region. Fifteen US pediatric training programs participated from June 14, 2017, to February 28, 2019. INTERVENTIONS: The intervention included 7 hour-long sessions of a monthly mindfulness curriculum (Mindfulness Intervention for New Interns) and a monthly mindfulness refresher implemented during internship. The active control arm included monthly 1-hour social lunches. MAIN OUTCOMES AND MEASURES: The primary outcome was emotional exhaustion (EE) as measured by the Maslach Burnout Inventory 9-question EE subscale (range, 7-63; higher scores correspond to greater perceived burnout). Secondary outcomes were depersonalization, personal accomplishment, and burnout. The study assessed mindfulness with the Five Facet Mindfulness Questionnaire and empathy with the Interpersonal Reactivity Index subscales of perspective taking and empathetic concern. Surveys were implemented at baseline, month 6, and month 15. RESULTS: Of the 365 interns invited to participate, 340 (93.2%; 255 [75.0%] female; 51 [15.0%] 30 years or older) completed surveys at baseline; 273 (74.8%) also participated at month 6 and 195 (53.4%) at month 15. Participants included 194 (57.1%) in the Mindfulness Intervention for New Interns and 146 (42.9%) in the control arm. Analyses were adjusted for baseline outcome measures. Both arms' EE scores were higher at 6 and 15 months than at baseline, but EE did not significantly differ by arm in multivariable analyses (6 months: 35.4 vs 32.4; adjusted difference, 3.03; 95% CI, -0.14 to 6.21; 15 months: 33.8 vs 32.9; adjusted difference, 1.42; 95% CI, -2.42 to 5.27). None of the 6 secondary outcomes significantly differed by arm at month 6 or month 15. CONCLUSIONS AND RELEVANCE: A novel mindfulness curriculum did not significantly affect EE, burnout, empathy, or mindfulness immediately or 9 months after curriculum implementation. These findings diverge from prior nonrandomized studies of mindfulness interventions, emphasizing the importance of rigorous study design and suggesting that additional study is needed to develop evidence-based methods to reduce trainee burnout. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT03148626.