Provided are a pharmaceutical use of poly-gamma-glutamic acid used for preventing or treating Th17-mediated diseases, a composition for preventing or treating Th17-mediated diseases comprising poly-gamma-glutamic acid as an active ingredient, and a method of preventing or treating Th17-mediated diseases using the same. The pharmaceutical composition is effective in inhibiting the differentiation of Th17 cells while promoting Treg cells, and, thus, can be useful in preventing or treating Th17-mediated diseases such as multiple sclerosis, rheumatoid arthritis, systemic lupus erythematosus, inflammatory bowel disease, Crohn's disease, ankylosing spondylitis, and the like.