Intervention studies have been undertaken to reduce sedentary behaviour (SB) and thereby potentially ameliorate unhealthy weight gain in children and adolescents. We synthesised evidence and quantified the effects of SB interventions (single or multiple components) on body mass index (BMI) or BMI z-score in this population. Publications up to March 2015 were located through electronic searches. Inclusion criteria were interventions targeting SB in children that had a control group and objective measures of weight and height. Mean change in BMI or BMI z-score from baseline to post-intervention were quantified for intervention and control groups and meta-analyzed using a random effects model. The pooled mean reduction in BMI and BMI z-score was significant but very small (standardized mean difference = -0.060, 95% confidence interval: -0.098 to -0.022). However, the pooled estimate was substantially greater for an overweight or obese population (standardized mean difference = -0.255, 95% confidence interval: -0.400 to -0.109). Multicomponent interventions (SB and other behaviours) delivered to children from 5 to 12 years old in a non-educational setting appear to favour BMI reduction. In summary, SB interventions are associated with very small improvement in BMI in mixed-weight populations. However, SB interventions should be part of multicomponent interventions for treating obese children.