OBJECTIVES: People aged 16-24 are more likely than other age groups to acquire sexually transmitted infections (STI). Safetxt was a randomised controlled trial of a theory-based digital health intervention to reduce STIs among 16-24 year-old people in the UK. We report results of qualitative research regarding participants' perceptions and experiences of the intervention and trial participation. DESIGN: Qualitative thematic analysis following a critical realist paradigm of written open feedback comments provided in the 12-month follow-up questionnaire and semistructured interviews. SETTING: Safetxt trial participants were recruited from UK sexual health clinics. PARTICIPANTS: Trial inclusion criteria: people aged 16-24 diagnosed with or treated for chlamydia, gonorrhoea or non-specific urethritis. Optional open feedback provided by 3526 of 6248 safetxt participants at 12 months and interviews with a purposive sample of 18 participants after the trial. RESULTS: We summarise and report results in seven broad themes. According to recipients, the safetxt intervention increased awareness of the importance of avoiding STIs and ways to prevent them. Participants reported improved confidence, agency, sexual well-being and communication about sexual health with partners, friends and family. Recipients attributed increased condom use, increased STI testing after (rather than before) sex with new partners, and more confident partner notification to the intervention. Recipients described a reduced sense of isolation and stigma in having an STI. Control group participants reported that having had an STI and receiving control texts asking them to report any changes in contact details acted as reminders to use condoms and get tested. We also summarise participant recommendations for future interventions and studies. CONCLUSIONS: While control group participants reported precautionary behaviours were 'triggered' by trial participation, intervention recipients reported additional benefits of the intervention in increasing precautionary behaviours and in broader aspects of sexual health such as confidence, communication, emotional well-being and agency. TRIAL REGISTRATION: ISRCTN registry ISRCTN64390461.