I've Been Killing Slimes for 300 Years and Maxed Out My Level (Japanese: スライム倒して300年、知らないうちにレベルMAXになってました) is a Japanese isekai light novel series written by Kisetsu Morita and illustrated by Benio. It was serialized online since 2016 on the user-generated novel publishing website Shōsetsuka ni Narō and, in the following year, it was acquired by SB Creative, who published the first light novel volume in 2017.[i] The light novel has been licensed in North America by Yen Press, who published the first volume in April 2018.[3] It has received a manga adaptation by Yūsuke Shiba in the same year as its debut of the light novel in Square Enix's Gangan Online website, also licensed by Yen Press. An anime television series adaptation by Revoroot aired from April to June 2021. A second season by Teddy is set to premiere in 2025. The series has also spawned various spin-off novels.