BackgroundThere is a critical global shortage of nurses in mental health, with workforce attrition due in large part to workplace stressors. Proactive strengths-based interventions to strengthen nurses' capacity to manage stress and improve mental health, wellbeing and resilience may also support workforce retention. ObjectiveTo determine the effects of a resilience-building programme on mental health nurses' coping self-efficacy (primary outcome), and psychological distress, wellbeing, resilience, posttraumatic growth, emotional intelligence behaviours, workplace belonging, and turnover intention (secondary outcomes). DesignPartially clustered randomised controlled trial. SettingLarge tertiary metropolitan mental health service in Australia. ParticipantsA total of 144 registered and enrolled nurses working clinically ≥0.6 full-time equivalent (73/intervention, 71/control), with 122 completing 3-month follow-up. MethodsThe Promoting Resilience in Nurses programme is an evidence-based workplace intervention delivered by trained facilitators across two workshops. Surveys were administered online upon registration and prior to randomisation (Time 1) into Intervention or Control (no intervention) arms, and immediately after the final workshop (Time 2), and at three months follow-up (Time 3). Linear mixed models for outcome measures were fitted to Time 2 and 3 responses. ResultsThere were seven intervention groups, with seven to 13 participants per group. Coping self-efficacy improved at Time 2 (estimated intervention effect 21.2 units, 95 % Confidence Intervals: 13.3 to 29.0) and Time 3 (12.1 units, 4.7 to 19.6), as well as wellbeing (Time 2: 9.2 units, 5.0 to 13.4), resilience (Time 2: 0.24 units, 0.01 to 0.46) and posttraumatic growth (Time 2: 16.1 units, 7.0 to 25.3). Psychological distress reduced (Time 2: −3.7 units, −6.2 to −1.31). All were sustained at three months. Emotional intelligence behaviours were improved (Time 2: 3.5 units, 0.6 to 6.5) but not sustained. Workplace belonging improved at Time 3 (0.34 units, 0.02 to 0.65) only. No statistically significant effects for turnover intention. ConclusionsDespite major contextual challenges, the Promoting Resilience in Nurses programme achieved the aims of promoting nurses' efficacy to cope with stress and regulate their emotions and improving mental health and wellbeing. The findings support the programme as a feasible and successful intervention for nurses across other settings and contexts. Trial registrationAustralian New Zealand Clinical Trials Registry (ACTRN12620001052921). Registered 15/10/2020. First recruitment 04/02/2021. Tweetable abstractPromoting Resilience in Nurses intervention improved coping self-efficacy, wellbeing, resilience, posttraumatic growth, emotional intelligence and psychological distress.