BACKGROUND: The SlowMo study demonstrated the effects of SlowMo, an eight-session digitally supported reasoning intervention, on paranoia in a large-scale randomized-controlled trial with 362 participants with schizophrenia-spectrum psychosis. AIM: The current evaluation aimed to investigate the impact of Patient and Public Involvement (PPI) in the SlowMo study. METHOD: PPI members were six women and three men from Sussex, Oxford and London with experience of using mental health services for psychosis. They received training and met at least 3-monthly throughout the project. The impact of PPI was captured quantitatively and qualitatively through (i) a PPI log of recommendations and implementation; (ii) written subjective experiences of PPI members; (iii) meeting minutes; and (iv) outputs produced. RESULTS: The PPI log revealed 107 recommendations arising from PPI meetings, of which 87 (81%) were implemented. Implementation was greater for recruitment-, data collection- and organization-related actions than for dissemination and emergent innovations. Qualitative feedback revealed impacts on study recruitment, data collection, PPI participants' confidence, knowledge, career aspirations and society more widely. Outputs produced included a film about psychosis that aired on BBC primetime television, novel webpages and journal articles. Barriers to PPI impact included geography, travel, funding, co-ordination and well-being. DISCUSSION: A future challenge for PPI impact will be the extent to which peer innovation (innovative PPI-led ideas) can be supported within research study delivery. PATIENT AND PUBLIC CONTRIBUTION: Planned Patient and Public Contribution in SlowMo comprised consultation and collaboration in (i) design, (ii) recruitment, (iii) qualitative interviews and analysis of service users' experiences of SlowMo therapy and (iv) dissemination.