BACKGROUND: The Mindfulness Ambassador Program (MAP) is a group-based, facilitated mindfulness-based intervention (MBI). We sought to determine the effectiveness of MAP on reducing negative psychotic symptoms and enhancing mindfulness skills among persons experiencing early psychosis. METHODS: We conducted a pragmatic randomized controlled trial (RCT) at three early psychosis intervention (EPI) programs in Ontario, Canada. Participants (N = 59) were randomly assigned to receive MAP (n = 29) for 1-hour weekly sessions over 3 months, or to treatment as usual (TAU, n = 30). Assessments were conducted at baseline, 3 months, and 6 months using the Self-Evaluation of Negative Symptoms (SNS) and Kentucky Inventory of Mindfulness Skills (KIMS). Linear mixed methods were used to assess the joint effects of group and time. RESULTS: At 3 months, participants who received MAP (n = 17) demonstrated greater reductions on the SNS relative to TAU (n = 15), which were clinically and statistically significant (-4.1; 95%CI -7.5, -0.7; p = 0.019). At 6 months, the difference between MAP (n = 10) and TAU (n = 13) was no longer statistically significant (-1.2; 95%CI -5.2,2.7; p = 0.54). On the KIMS, no significant effects were found at 3 months (+0.3; 95%CI -2.0,2.5; p = 0.82) or 6 months (+0.4; 95%CI -2.2,2.9; p = 0.79). CONCLUSIONS: We conducted one of the first multi-site RCTs of a MBI for early psychosis. Our findings indicated that MAP was more effective in reducing negative symptoms compared to TAU in the short term. Earlier reductions in negative psychotic symptoms may help facilitate recovery in the long term.