By reading relevant reviews and summarizing a large number of basic and clinical studies, it was found that, although the current studies on IL-17 have been more extensively and comprehensively elucidated in several neurological diseases and mental disorders, unlike a previous study on IL-17 that has been mostly limited to mechanisms such as neuroinflammatory effects, our review additionally adopts the microbiota–gut–brain axis as a novel perspective to discuss how IL-17 directly or indirectly influences the immune system in physiological and pathological contexts. This evidence concerns the gene IL17A and mental disorder.