BACKGROUND: A Growing body of literature indicates therapeutic effectiveness of mindfulness for mental disorders. Only few trials have been conducted with schizophrenia spectrum disorders (SSD), mostly in outpatient settings. Primary objective was to assess feasibility, acceptability, and preliminary outcomes of mindfulness-based group therapy (MBGT) for in-patients with SSD. METHODS: A pre-registered randomized controlled trial was conducted to assess feasibility and acceptability of the MBGT. The primary outcome was mindfulness measured with the Southampton Mindfulness Questionnaire (SMQ). Secondary outcomes were rater-blinded positive- and negative symptoms, depression, social functioning, and self-reported mindfulness, depression, anxiety, psychological flexibility, quality of life, and medication regime at baseline, post-intervention, and follow-up (Clinical Trails NCT03671005). RESULTS: 40 participants received either treatment-as-usual (TAU; n=19) or (MBGT+TAU; n = 21) for four weeks. At post-intervention, protocol adherence was 95.2%, and retention rate was 95%. ANCOVA revealed significant improvements in the MBGT+TAU for the primary outcome SMQ as well as negative symptoms at post-intervention between groups. In exploratory analyses, secondary outcomes showed medium-to-large pre-to-post-intervention effects on mindfulness, positive-, negative-, and depressive symptoms, psychological flexibility, quality of life, and social functioning for MBGT+TAU and small-to-moderate changes on positive symptoms and social functioning for TAU. No serious adverse effects were reported. CONCLUSIONS: MBGT appears feasible and acceptable for in-patient settings, with high protocol adherence and retention rates. Preliminary findings highlight a proof of concept of MBGT and various improvements in clinical- and process dimensions. A fully powered trial is warranted to determine efficacy, cost-efficiency, and longitudinal changes based on these promising outcomes.