Nonalcoholic fatty liver disease (NAFLD) is the most common and important chronic liver disease all over the world. In the present study, we found that koumine, the main and active ingredient isolated from Gelsemium elegans, has the potential therapeutic effect on NAFLD rats by immunomodulatory activity. Koumine could significantly reduce the level of TG, TC, LDL-C, ALT, and AST in the serum of NAFLD rats and increase the level of HDL-C, reduce the liver index, and improve the adipose-like lesions of liver cells in NAFLD rats. Furthermore, treatment with koumine inhibited the severity of NAFLD. In addition, koumine-treated rats significantly increased the proportion of CD4+/CD8+ T cells and also decreased the percentages of Th1 and Th17 cells and increased Th2 and Treg cells in the liver. Moreover, koumine reduced the production and mRNA expression of proinflammatory cytokines in vivo. This result showed that koumine could effectively modulate different subtypes of helper T cells and prevent NAFLD. The present study revealed the novel immunomodulatory activity of koumine and highlighted the importance to further investigate the effects of koumine on hepatic manifestation of the metabolic syndrome.