α-Lipoic acid (ALA) has been reported to be effective in reducing body weight in rodents and obese patients. Our previous open trial showed that ALA may play a role in reducing weight gain in patients with schizophrenia on atypical antipsychotics. The present study evaluated the efficacy of ALA in reducing weight and BMI in patients with schizophrenia who had experienced significant weight gain since taking atypical antipsychotics. In a 12-week, double-blind randomized placebo-controlled study, 22 overweight and clinically stable patients with schizophrenia were randomly assigned to receive ALA or placebo. ALA was administered at 600–1800 mg, as tolerated. Weight, BMI, abdomen fat area measured by computed tomography, and metabolic values were determined. Adverse effects were also assessed to examine safety. Overall, 15 patients completed 12 weeks of treatment. There was significant weight loss and decreased visceral fat levels in the ALA group compared with the placebo group. There were no instances of psychopathologic aggravation or severe ALA-associated adverse effects. ALA was effective in reducing weight and abdominal obesity in patients with schizophrenia who had experienced significant weight gain since beginning an atypical antipsychotic regimen. Moreover, ALA was well tolerated throughout this study. ALA might play an important role as an adjunctive treatment in decreasing obesity in patients who take atypical antipsychotics.