That Time I Got Reincarnated as a Slime (転生したらスライムだった件 Tensei Shitara Slime Datta Ken) is a light novel series written by Fuse and illustrated by Mitz Vah. It was serialized online between 2013 and 2016 on Shōsetsuka ni Narō. The series was acquired by Micro Magazine, who published the first volume in May 2014 under their GC Novels imprint. Yen Press licensed the novels for North American release and published the first volume in December 2017.
The light novel series has over 4.5 million volumes in print and was the fifth best-selling title of 2018 with 539,277 copies sold. The novel ranked eighth in 2017 in Takarajimasha's annual light novel guide book Kono Light Novel ga Sugoi!, in the tankōbon category. It ranked sixth in 2018 and fifth in 2019.