Pelvic organ prolapse is common, causes unpleasant symptoms and negatively affects women's quality of life. In the UK, most women with pelvic organ prolapse attend clinics for pessary care. To determine the clinical effectiveness and cost-effectiveness of vaginal pessary self-management on prolapse-specific quality of life for women with prolapse compared with clinic-based care; and to assess intervention acceptability and contextual influences on effectiveness, adherence and fidelity. A multicentre, parallel-group, superiority randomised controlled trial with a mixed-methods process evaluation. Women attending UK NHS outpatient pessary services, aged ≥ 18 years, using a pessary of any type/material (except shelf, Gellhorn or Cube) for at least 2 weeks. Exclusions: women with limited manual dexterity, with cognitive deficit (prohibiting consent or self-management), pregnant or non-English-speaking. The self-management intervention involved a 30-minute teaching appointment, an information leaflet, a 2-week follow-up telephone call and a local clinic telephone helpline number. Clinic-based care involved routine appointments determined by centres' usual practice. Remote web-based application; minimisation was by age, pessary user type and centre. Participants, those delivering the intervention and researchers were not blinded to group allocation. The patient-reported primary outcome (measured using the Pelvic Floor Impact Questionnaire-7) was prolapse-specific quality of life, and the cost-effectiveness outcome was incremental cost per quality-adjusted life-year (a specifically developed health Resource Use Questionnaire was used) at 18 months post randomisation. Secondary outcome measures included self-efficacy and complications. Process evaluation data were collected by interview, audio-recording and checklist. Analysis was by intention to treat. Three hundred and forty women were randomised (self-management, n = 169; clinic-based care, n = 171). At 18 months post randomisation, 291 questionnaires with valid primary outcome data were available (self-management, n = 139; clinic-based care, n = 152). Baseline economic analysis was based on 264 participants (self-management, n = 125; clinic-based care, n = 139) with valid quality of life and resource use data. Self-management was an acceptable intervention. There was no group difference in prolapse-specific quality of life at 18 months (adjusted mean difference -0.03, 95% confidence interval -9.32 to 9.25). There was fidelity to intervention delivery. Self-management was cost-effective at a willingness-to-pay threshold of £20,000 per quality-adjusted life-year gained, with an estimated incremental net benefit of £564.32 and an 80.81% probability of cost-effectiveness. At 18 months, more pessary complications were reported in the clinic-based care group (adjusted mean difference 3.83, 95% confidence interval 0.81 to 6.86). There was no group difference in general self-efficacy, but self-managing women were more confident in pessary self-management activities. In both groups, contextual factors impacted on adherence and effectiveness. There were no reported serious unexpected serious adverse reactions. There were 32 serious adverse events (self-management, n = 17; clinic-based care, n = 14), all unrelated to the intervention. Skew in the baseline data for the Pelvic Floor Impact Questionnaire-7, the influence of the global COVID-19 pandemic, the potential effects of crossover and the lack of ethnic diversity in the recruited sample were possible limitations. Self-management was acceptable and cost-effective, led to fewer complications and did not improve or worsen quality of life for women with prolapse compared with clinic-based care. Future research is needed to develop a quality-of-life measure that is sensitive to the changes women desire from treatment. This study is registered as ISRCTN62510577. This award was funded by the National Institute for Health and Care Research (NIHR) Health Technology Assessment programme (NIHR award ref: 16/82/01) and is published in full in Health Technology Assessment; Vol. 28, No. 23. See the NIHR Funding and Awards website for further award information.