Objective A previous study reported on a 16-week placebo-controlled, randomized clinical trial (RCT) of metformin for weight stabilization in 61 children and adolescents 6 to 17 years old with autism spectrum disorder who were prescribed atypical antipsychotics. The present study describes the results of a 16-week open-label extension. Method Fifty-two participants from the acute trial (85%) entered the extension; 22 had been on metformin during the initial RCT and 30 had been on placebo. Participants were re-titrated to 500 mg twice a day (6- to 9-year-olds) or 850 mg twice a day (10- to 17-year-olds) during the open-label extension. Primary outcome measure was change in body mass index (BMI) z-score after 16 weeks; secondary outcomes were change in additional body composition and metabolic parameters. Results After 16 weeks of open-label treatment, participants initially taking placebo during the RCT had lower BMI z-scores (mean 16-week change −0.10, p =.004). Statistically significant improvements also were noted in secondary body composition measures (weight z-score and BMI and weight percentile) but not in metabolic variables. Participants who initially had been taking metformin during the 16-week RCT maintained prior decreases in BMI z-scores but did not have additional weight loss. Three participants discontinued treatment because of an adverse event. No significant changes were noted on metabolic measures, although the decrease in hemoglobin A1c was large (∼1 mmol) and consistent across the acute and open-label phases. Conclusion Metformin can be effective for decreasing weight gain associated with atypical antipsychotic use and maintaining prior improvement in children and adolescents with autism spectrum disorder. Clinical trial registration information—Treatment of Overweight Induced by Antipsychotic Medication in Young People With Autism Spectrum Disorders (ASD); http://clinicaltrials.gov/; NCT01825798.