Non-alcoholic fatty liver disease (NAFLD) has been on a global rise. While animal models have rendered valuable insights to the pathogenesis of NAFLD, discrepancy with patient data still exists. Since non-alcoholic steatohepatitis (NASH) involves chronic inflammation, and CD4+ T cell infiltration of the liver is characteristic of NASH patients, we established and characterized a humanized mouse model to identify human-specific immune response(s) associated with NAFLD progression. Immunodeficient mice engrafted with human immune cells (HIL mice) were fed with high fat and high calorie (HFHC) or chow diet for 20 weeks. Liver histology and immune profile of HIL mice were analyzed and compared with patient data. HIL mice on HFHC diet developed steatosis, inflammation and fibrosis of the liver. Human CD4+ central and effector memory T cells increased within the liver and in the peripheral blood of our HIL mice, accompanied by marked up-regulation of pro-inflammatory cytokines (IL-17A and IFNγ). In vivo depletion of human CD4+ T cells in HIL mice reduced liver inflammation and fibrosis, but not steatosis. Our results highlight CD4+ memory T cell subsets as important drivers of NAFLD progression from steatosis to fibrosis and provides a humanized mouse model for pre-clinical evaluation of potential therapeutics. Agency for Science, Technology and Research (A*STAR) National Medical Research Council (NMRC) National Research Foundation (NRF) Published version This study was supported by the National Research Foundation Singapore Fellowship (NRF-NRFF2017-03), NRF-ISF joint grant (NRF2019-NRF-ISF003-3127), Ensemble of Multi-Disciplinary Systems and Integrated Omics for NAFLD (EMULSION) diagnostic and therapeutic discovery (H18/01/a0/017), Agency for Science, Technology and Research (A∗STAR), Gilead Sciences International Research Scholars Program in Liver Disease (to QC), National Natural Science Foundation of China (81970520), and National Medical Research Council – Clinician Scientist – Individual Research Grant (NMRC/CIRG/1427/2015).