Objective To carry out a randomised controlled trial on the effect of a new method of health promotion-email and mobile phone text messages (short messaging service (SMS))-on young people’s sexual health. Methods 994 people aged 16-29 were recruited at a music festival to a non-blinded randomised controlled trial. Participants were randomised to either receive sexual health promotion messages (n=507) or the control group (n=487). The 12-month intervention included SMS (catchy sexually transmissible infections prevention slogans) and emails. Participants completed questionnaires at the festival at baseline and online after 3, 6 and 12 months. Outcomes were differences between the control and intervention groups in health-seeking behaviour, condom use with risky partners (new or casual partners or two or more partners within 12 months) and STI knowledge. Results 337 (34%) completed all three follow-up questionnaires and 387 (39%) completed the final questionnaire. At 12 months, STI knowledge was higher in the intervention group for both male (OR=3.19 95% CI 1.52 to 6.69) and female subjects (OR=2.36 95% CI 1.27 to 4.37). Women (but not men) in the intervention group were more likely to have had an STI test (OR=2.51, 95% CI 1.11 to 5.69), or discuss sexual health with a clinician (OR=2.92, 95% CI 1.66 to 5.15) than their control counterparts. There was no significant impact on condom use. Opinions of the messages were favourable. Conclusion This simple intervention improved STI knowledge in both sexes and STI testing in women, but had no impact on condom use. SMS and email are low cost, popular and convenient, and have considerable potential for health promotion. Clinical trial registration number Australian Clinical Trials Registry - ACTRN12605000760673.