Background: Dipeptidyl peptidase 4 (DPP4) is a serine exopeptidase able to inactivate various oligopeptides and also a hepatokine. Hepatocyte-specific overexpression of DPP4 is associated with hepatic insulin resistance and liver steatosis. Method: We examined whether weekly DPP4 inhibitor omarigliptin (OMG) improves liver function as well as levels of inflammation and insulin resistance in type 2 diabetic patients with non-alcoholic fatty liver disease (NAFLD). Furthermore, we tried OMG in a diabetic patient with biopsy-confirmed nonalcoholic steatohepatitis (NASH). Results: In NAFLD patients, OMG significantly decreased levels of aminotransferase (ALT), aspartate aminotransferase (AST), gamma-glutamyl transpeptidase (gGTP), homeostatic model assessment of insulin resistance (HOMA-IR), and high-sensitivity C-reactive protein (hsCRP), while no significant change was seen in hemoglobin A1c (HbA1c) or body mass index (BMI). In a NASH patient, liver function had improved markedly, and the hepatic fibrosis marker FIB-4 decreased in parallel with HOMA-IR and hsCRP. Improvements in intrahepatic fat deposition and fibrosis appeared to be seen on ultrasonography.Conclusion: The effects of OMG in ameliorating hepatic insulin resistance may lead to decreasing intrahepatic fat accumulation and improving intrahepatic adipose inflammation in NAFLD/NASH.Trial registration: UMIN Clinical Registry (UMIN000029288). Registered 22 September, 2017, https://upload.umin.ac.jp/UMIN000029288