BACKGROUND: Younger people bear the heaviest burden of sexually transmitted infections (STIs). Partner notification, condom use and STI testing can reduce infection but many young people lack the knowledge, skills and confidence needed to carry out these behaviours. Text messages can provide effective behavioural support. The acceptability and feasibility of a randomised controlled trial of safer sex support delivered by text message are not known. OBJECTIVES: To assess the acceptability and feasibility of a randomised controlled trial of a safer sex intervention delivered by text message for young people aged 16-24 years. DESIGN: (1) Intervention development; (2) follow-up procedure development; (3) a pilot, parallel-arm randomised controlled trial with allocation via remote automated randomisation (ratio of 1 : 1) (participants were unmasked, whereas researchers analysing samples and data were masked); and (4) qualitative interviews. SETTING: Participants were recruited from sexual health services in the UK. PARTICIPANTS: Young people aged 16-24 years diagnosed with chlamydia or reporting unprotected sex with more than one partner in the last year. INTERVENTIONS: A theory- and evidence-based safer sex intervention designed, with young people's input, to reduce the incidence of STIs by increasing the correct treatment of STIs, partner notification, condom use and STI testing before unprotected sex with a new partner. The intervention was delivered via automated mobile phone messaging over 12 months. The comparator was a monthly text message checking contact details. MAIN OUTCOME MEASURES: (1) Development of the intervention based on theory, evidence and expert and user views; (2) follow-up procedures; (3) pilot trial primary outcomes: full recruitment within 3 months and follow-up rate for the proposed primary outcomes for the main trial; and (4) participants' views and experiences regarding the acceptability of the intervention. RESULTS: In total, 200 participants were randomised in the pilot trial, of whom 99 were allocated to the intervention and 101 were allocated to the control. We fully recruited early and achieved an 81% follow-up rate for our proposed primary outcome of the cumulative incidence of chlamydia at 12 months. There was no differential follow-up between groups. In total, 97% of messages sent were successfully delivered to participants' mobile phones. Recipients reported that the tone, language, content and frequency of messages were appropriate. Messages reportedly increased knowledge of and confidence in how to use condoms and negotiate condom use and reduced stigma about STIs, enabling participants to tell a partner about a STI. CONCLUSIONS: Our research shows that the intervention is acceptable and feasible to deliver. Our pilot trial demonstrated that a main trial is feasible. It remains unclear which behaviour change techniques and elements of the intervention or follow-up procedures are associated with effectiveness. A further limitation is that in the trial one person entering data and the participants were unmasked. A randomised controlled trial to establish the effects of the intervention on STIs at 12 months is needed. TRIAL REGISTRATION: Current Controlled Trials ISRCTN02304709. 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. 20, No. 57. See the NIHR Journals Library website for further project information.