Sexual health promotion is a major public health challenge. There is huge potential for health promotion via technology such as the Internet, but it is not known whether interventions are effective. An interactive computer-based intervention provides information, and also offers personalised feedback. We searched databases for studies which were randomised controlled trials (RCTs) of computer/Internet-based interventions which aimed to improve sexual health. We included trials of computer-based interventions delivered to people of any age, gender, sexual orientation, ethnicity or nationality. The review evaluated 15 RCTs involving 3917 participants. Results showed that computer-based interventions have a moderate effect in improving people's knowledge about sexual health in comparison to minimal interventions such as ‘usual practice' or a leaflet. We also found a small effect on safer sex self-efficacy (a person's belief in their capacity to carry out a specific action), a small effect on safer-sex intentions, and also an effect on sexual behaviour (such as condom use for sexual intercourse). We found that computer-based interventions seem better than face-to-face interventions at improving sexual health knowledge, but there were insufficient data to analyse other outcomes. No studies measured potential harms (apart from reporting any deterioration in outcomes). Interactive computer-based interventions for sexual health promotion are feasible in a variety of settings. They are effective tools for learning about sexual health, and they also improve self-efficacy, intention and sexual behaviour, but more research is needed to establish whether computer-based interventions can change outcomes such as sexually transmitted infections and pregnancy, to understand how interventions might work, and to assess whether they are cost-effective.