Background The use of mobile technologies to prevent STIs is recognised as a promising approach worldwide; however, evidence has been inconclusive, and the field has developed rapidly. With about 1 million new STIs a day globally, up-to-date evidence is urgently needed.

Objective To assess the effectiveness of mobile health interventions delivered to participants for preventing STIs and promoting preventive behaviour.

Methods We searched seven databases and reference lists of 49 related reviews (January 1990–February 2020) and contacted experts in the field. We included randomised controlled trials of mobile interventions delivered to adolescents and adults to prevent sexual transmission of STIs. We conducted meta-analyses and assessed risk of bias and certainty of evidence following Cochrane guidance.

Results After double screening 6683 records, we included 22 trials into the systematic review and 20 into meta-analyses; 18 trials used text messages, 3 used smartphone applications and 1 used Facebook messages as delivery modes. The certainty of evidence regarding intervention effects on STI/HIV occurrence and adverse events was low or very low. There was moderate certainty of evidence that in the short/medium-term text messaging interventions had little or no effect on condom use (standardised mean differences (SMD) 0.02, 95% CI −0.09 to 0.14, nine trials), but increased STI/HIV testing (OR 1.83, 95% CI 1.41 to 2.36, seven trials), although not if the standard-of-care control already contained an active text messaging component (OR 1.00, 95% CI 0.68 to 1.47, two trials). Smartphone application messages also increased STI/HIV testing (risk ratio 1.40, 95% CI 1.22 to 1.60, subgroup analysis, two trials). The effects on other outcomes or of social media or blended interventions is uncertain due to low or very low certainty evidence.

Conclusions Text messaging interventions probably increase STI/HIV testing but not condom use in the short/medium term. Ongoing trials will report the effects on biological and other outcomes.