We reviewed web-based interventions for overweight and obesity prevention. A literature search was conducted using seven electronic databases. Manually searched articles were also included. Thirty studies fulfilled the inclusion criteria. Of these, 13 studied physical activity, eight studied dietary practices and nine studied a combination of physical activity and dietary practice. Twenty-eight of the studies (93%) reported positive changes in moderate to vigorous physical activity level, fruit and vegetable intake and psychological factors. A meta-analysis showed there were improvements, though not significant, in fruit and vegetable consumption (standardised mean difference, SMD = 0.61; 95% CI =−0.13 to 1.35) and physical activity (SMD = 0.15; 95% CI =−0.06 to 0.35). The review suggests that web-based interventions are a useful educational tool for increasing awareness and making healthy behaviour changes in relation to an excessive weight gain problem.