This systematic review of RCTs aimed to determine whether or not chitosan, a popular OTC weight-loss supplement, is an effective treatment for overweight and obesity. Included were trials of chitosan with a minimum duration of 4 weeks in adults who were overweight or obese and/or had hypercholesterolaemia at baseline. Fourteen trials involving a total of 1071 participants were included. Analyses involving all trials indicated that chitosan preparations result in a small but statistically significant greater reduction in body-weight (weighted mean difference −1.7 kg; 95% CI −2.1 to −1.3 kg) compared with placebo. Analyses restricted to high-quality studies showed that reductions in weight [−0.6 (−1.2 to 0.1) kg] were less than in lower quality studies [−2.3 (−2.7 to −1.8)kg]. Results obtained from high-quality trials indicate that the effect of chitosan on body-weight is minimal and unlikely to be of clinical significance.