Bariatric surgery has proven to be the most effective treatment for controlling hyperglycemia in severely obese patients with diabetes. We show that fibroblast growth factor 19 (FGF19), a gut hormone, is rapidly induced by bariatric surgery in rodents and humans. Administration of FGF19 achieves diabetes remission independent of weight loss in animal models of diabetes, supporting a role for FGF19 in the hormonal remodeling that restores metabolic function after the surgery. Through an unbiased, systematic screen in diabetic mice, we identified selective, safe, and effective FGF19 analogs. Unexpectedly, a lead FGF19 analog, NGM282, did not correct hyperglycemia in patients with type 2 diabetes. In contrast, administration of NGM282 resulted in a rapid, robust, and sustained reduction in liver fat content and an improvement in liver histology in patients with nonalcoholic steatohepatitis, faithfully replicating another key benefit of bariatric surgery. Our work identifies a strategy for replacing the surgery with an equally effective, but less invasive, treatment for nonalcoholic steatohepatitis.