A 24-year-old man was admitted to our hospital because of liver dysfunction. He had been diagnosed as having psoriasis vulgaris at 18 years of age. Physical examination demonstrated obesity, general erythema, and hepatomegaly. Laboratory data revealed elevated serum levels of aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, and glucose. A histological examination of the liver revealed macrovesicular fatty change and infiltration of inflammatory cells, including lymphocytes and polymorphonuclear cells, within the liver lobules. Pericentral fibrosis and pericellular fibrosis were also recognized. He was diagnosed as having nonalcoholic steatohepatitis (NASH), based on the fact that he had no habit of drinking alcohol, as well as psoriasis vulgaris and diabetes mellitus. We herein report a very rare case of NASH associated with psoriasis vulgaris.