Web-based interventions can be effective in changing behaviour of people faced with health problems. However, it is unclear whether they are effective in preventing health problems like overweight. The aim of this study was to investigate usage and effectiveness of the Healthy Weight Assistant (HWA), a web-based application to increase healthy behaviour in adults with a healthy weight or slight overweight, by means of a Randomised Controlled Trial (RCT). 297 respondents were randomly assigned to the intervention (n=147) or the waiting list group (n=150). The intervention group received access to the intervention for 12 weeks. At pre- and post-test we measured dietary and physical activity behaviour (primary outcomes) and BMI, knowledge, attitude, self-efficacy, subjective behaviour and insight in behaviour (secondary outcomes). All participants, regardless of group, show improvement in healthy behaviour and subjective assessment of healthy behaviour. People who are older, score higher on dietary behaviour and under-estimate their dietary behaviour are more likely to use the HWA. Using the HWA leads to improvement in physical activity behaviour and insight in physical activity behaviour.