Background Education is broader than academic teaching. It includes teaching students social-emotional skills both directly and indirectly through a positive school climate. Objective To evaluate if a universal school-based mindfulness training (SBMT) enhances teacher mental health and school climate. Methods The My Resilience in Adolescence parallel group, cluster randomised controlled trial (registration: ISRCTN86619085; funding: Wellcome Trust (WT104908/Z/14/Z, WT107496/Z/15/Z)) recruited 85 schools (679 teachers) delivering social and emotional teaching across the UK. Schools (clusters) were randomised 1:1 to either continue this provision (teaching as usual (TAU)) or include universal SBMT. Data on teacher mental health and school climate were collected at prerandomisation, postpersonal mindfulness and SBMT teacher training, after delivering SBMT to students, and at 1-year follow-up. Finding Schools were recruited in academic years 2016/2017 and 2017/2018. Primary analysis (SBMT: 43 schools/362 teachers; TAU: 41 schools/310 teachers) showed that after delivering SBMT to students, SBMT versus TAU enhanced teachers' mental health (burnout) and school climate. Adjusted standardised mean differences (SBMT minus TAU) were: exhaustion (-0.22; 95% CI -0.38 to -0.05); personal accomplishment (-0.21; -0.41, -0.02); school leadership (0.24; 0.04, 0.44); and respectful climate (0.26; 0.06, 0.47). Effects on burnout were not significant at 1-year follow-up. Effects on school climate were maintained only for respectful climate. No SBMT-related serious adverse events were reported. Conclusions SBMT supports short-term changes in teacher burnout and school climate. Further work is required to explore how best to sustain improvements. Clinical implications SBMT has limited effects on teachers' mental and school climate. Innovative approaches to support and preserve teachers' mental health and school climate are needed.