The official website of A Certain Magical Index announced that a third season is green-lit for 2018. A new preview and visual announcement was also released. The anime will premiere as part of A Certain Project 2018.
J.C. Staff will be returning to animate the third season of the series. ANN reports that the voice actor of Toma Kamijo, Atsuhi Abe and the voice actor of Index, Yuka Iguchi, will be returning to their roles for the new season.
— とあるプロジェクト公式 (@toaru_project) October 1, 2017