Yesterday at the “Shingeki Matsuri” event, Attack on Titan Season 2 was announced to premiere in spring 2017. A new image promoting the new season was released, along with the cast for the second season.

originally the second season was announced way back in 2014, and was supposed to premiere in 2016, but was delayed due to creator wanting to get the manga a lot further ahead of the anime.
Be sure to check the teaser below that was originally shown earlier this year.