The efficacy of a tailored, web-based intervention communicating the risks of sexually transmitted infections (STI) for heterosexual young adults was examined in a randomised, controlled trial. The main aims of the relationship-oriented intervention were to influence risk perceptions and to promote (maintenance of) condom use and STI-testing among young adults who reported being recently engaged in a heterosexual relationship. The intervention addressed risk perceptions, attitudes, normative beliefs, self-efficacy and skills related to condom use and STI-testing. Outcomes were compared immediately after the intervention (N = 171) and 3 months later (N = 115) to a non-tailored intervention group and to a control group. Cognitive and behavioural outcomes showed that the tailored intervention was efficacious in influencing perceived susceptibility to STI and STI-testing intentions immediately after the intervention, and in reducing rates of unprotected sex at 3 months.