Background People with serious mental illness have sexual health needs, but there is limited evidence regarding effective interventions to promote their sexual health. Objectives To develop a sexual health promotion intervention for people with serious mental illness, and to conduct a feasibility trial in order to establish the acceptability and parameters for a fully powered trial. Design A two-armed randomised controlled, open feasibility study comparing usual care alone with usual care plus the adjunctive intervention. Setting Five community mental health providers in Leeds, Barnsley, Brighton and London. Participants Adults aged ≥ 18 years with serious mental illness and receiving care from community mental health teams. Interventions A remote, web-based computer randomisation system allocated participants to usual care plus the RESPECT (Randomised Evaluation of Sexual health Promotion Effectiveness informing Care and Treatment) intervention (three sessions of 1 hour) (intervention arm) or usual care only (control arm). The intervention was an interactive manualised package of exercises, quizzes and discussion topics focusing on knowledge, motivation and behavioural intentions to adopt safer sexual behaviours. Main outcome measures Feasibility parameters including establishing the percentage of people who were eligible, consented and were retained in each arm of the trial, retention for the intervention, as well as the completeness of the data collection. Data were collected on knowledge, motivation to adopt safer sexual behaviour, sexual behaviour, sexual stigma, sexual health service use and quality of life. Data were collected at baseline and then at 3 months and 6 months post randomisation. Results Of a target of 100 participants, 72 people participated in the trial over 12 months. Of the 36 participants randomised to the intervention arm, 27 received some of the intervention (75.0%). At 3 months, 59 of the 72 participants completed follow-up questionnaires (81.9%) (30 participants from the intervention arm and 29 participants from the control arm). Only the first 38 participants were followed up at 6 months. However, data were collected on 29 out of 38 participants (76.3% retention): 13 in the intervention arm and 16 in the control arm. No adverse events were reported. Participant feedback confirmed that both the design and the intervention were acceptable. The economic analysis indicated high completion rates and completeness of data among participants who continued the trial. Conclusions Despite the limitations, the findings suggest that it is both acceptable and feasible to undertake a sexual health promotion study for people with serious mental illness. Future work A fully powered randomised controlled trial would be required to establish the clinical effectiveness of the intervention. Trial registration Current Controlled Trials ISRCTN15747739. Funding This project was funded by the National Institute for Health Research (NIHR) Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 23, No. 65. See the NIHR Journals Library website for further project information.