Strategies for prevention and treatment of nonalcoholic steatohepatitis (NASH) remain to be established. Here, we evaluated the effect of glycine on metabolic steatohepatitis in genetically obese, diabetic KK-A^y mice. Male KK-A^y mice were fed a diet containing 5% glycine for 4 weeks, and liver pathology was evaluated. Hepatic mRNA levels for lipid-regulating molecules, cytokines/chemokines, and macrophage M1/M2 markers were determined by real time RT-PCR. Hepatic expression of natural killer (NK) T cells was analyzed by flow cytometry. Dietary glycine blunted body weight gain significantly, and prevented the development of hepatic steatosis and inflammatory infiltration remarkably as compared to controls. Indeed, hepatic induction levels of molecules related to lipogenesis were largely blunted in glycine-fed mice. Dietary glycine also blunted elevations in hepatic mRNA levels for TNFα and chemokine (C-C motif) ligand (CCL)2 remarkably in these mice. Further, dietary glycine reversed suppression of hepatic NKT cells in KK-A^y mice, which was accompanied by increases in basal hepatic expression levels of NKT cell-derived cytokines such as IL-4 and IL-13. Moreover, hepatic mRNA levels of arginase-1, a marker of macrophage M2 transformation, were significantly increased in glycine-fed mice. Furthermore, dietary glycine improved glucose tolerance and hyperinsulinemia in KK-A^y mice. These observations clearly indicate that glycine prevents maturity-onset obesity and metabolic steatohepatitis in genetically diabetic KK-A^y mice. The underlying mechanisms most likely include normalization in hepatic innate immune responses involving NKT cells and M2 transformation of Kupffer cells. It is proposed that glycine is a promising immuno-nutrient for prevention and treatment of metabolic syndrome-related NASH.