OBJECTIVE: Lung cancer patients report among the highest distress rates of all cancer patients. Partners report similar distress rates. The present study examined the effectiveness of additional mindfulness-based stress reduction (care as usual [CAU] + MBSR) versus solely CAU to reduce psychological distress in lung cancer patients and/or their partners. METHODS: We performed a multicentre, parallel-group, randomized controlled trial. Mindfulness-based stress reduction is an 8-week group-based intervention, including mindfulness practice and teachings on stress. Care as usual included anticancer treatment, medical consultations, and supportive care. The primary outcome was psychological distress. Secondary outcomes included quality of life, caregiver burden, relationship satisfaction, mindfulness skills, self-compassion, rumination, and posttraumatic stress symptoms. Outcomes were assessed at baseline, post-intervention, and 3-month follow-up. Linear mixed modeling was conducted on an intention-to-treat sample. Moderation (gender, disease stage, baseline distress, participation with/without partner) and mediation analyses were performed. RESULTS: A total of 31 patients and 21 partners were randomized to CAU + MBSR and 32 patients and 23 partners to CAU. After CAU + MBSR patients reported significantly less psychological distress (p = .008, d = .69) than after CAU. Baseline distress moderated outcome: those with more distress benefitted most from MBSR. Additionally, after CAU + MBSR patients showed more improvements in quality of life, mindfulness skills, self-compassion, and rumination than after CAU. In partners, no differences were found between groups. CONCLUSION: Our findings suggest that psychological distress in lung cancer patients can be effectively treated with MBSR. No effect was found in partners, possibly because they were more focused on patients' well-being rather than their own.