BACKGROUND Bariatric surgery is effective for the treatment of patients with morbid obesity and type 2 diabetes mellitus (T2DM), for body weight loss and glycemic control. However, in Japan, there has been no previous report of the effectiveness bariatric surgery in a case of morbid obesity associated with acute onset type 1 diabetes mellitus (T1DM), in which pancreatic β-cells were destroyed and endogenous insulin was depleted. METHODS A 36-year-old woman with morbid obesity and T1DM, diagnosed when she was 6 years, was admitted for bariatric surgery. At her first consultation, she had a body weight of 106.7 kg and a body mass index of 42.2 kg/m2. Her HbA1c level was 9.0%, with a required daily insulin dose of 75 units. She underwent laparoscopic sleeve gastrectomy. At 1 year after surgery, her body weight had decreased to 81.0 kg and her body mass index to 32.2 kg/m2. In addition, her daily required dose of insulin had decreased to 24 units, with an improvement in her HbA1c level to 7.7%. CONCLUSIONS Although further evidence needs to be accumulated, including long-term outcomes, laparoscopic sleeve gastrectomy may provide an effective treatment for patients with morbid obesity and T1DM for body weight loss, improvement in HbA1c level, and insulin dose reduction.