Abstract Background Few studies have examined the economic costs of outpatient care for eating disorders in Japan. This study aimed to clarify the reimbursement for outpatient treatment of eating disorders and compare the costs between the departments of Psychosomatic Medicine and Psychiatry in Japan. Method A multicenter, prospective, observational study of patients with an eating disorder was conducted in the Psychosomatic Medicine departments of three centers and the Psychiatry departments of another three centers in Japan. We analyzed medical reimbursement for an outpatient revisit, time of clinical interviews, and the treatment outcome measured by the Eating Disorder Examination Questionnaire (EDE-Q) global scores and body mass index (BMI) at 3 months. Multivariate linear regression models were performed to adjust for covariates. Results This study included 188 patients in the Psychosomatic Medicine departments and 68 in the Psychiatry departments. The average reimbursement cost for an outpatient revisit was 4670 yen. Even after controlling for covariates, the Psychosomatic Medicine departments had lower reimbursement points per minute of interviews than the Psychiatry departments (coefficient = − 23.86; 95% confidence interval = − 32.09 to − 15.63; P