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, Hepburn: Suraimu Taoshite Sanbyaku-nen, Shiranai Uchi ni Reberu Makkusu ni Nattemashita) is a Japanese isekai light novel series written by Kisetsu Morita and illustrated by Benio.

[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.

She finds herself reincarnated as an undying, unaging witch in a new world, and now she vows to spend her days as pleasantly and stress-free as possible.

Originally, the series was published by Kisetsu Morita on the user-generated web novel site Shōsetsuka ni Narō in 2016.

In September 2019, those stories were compiled into its own volume and the spin-off, called I Was a Bottom-Tier Bureaucrat for 1,500 Years, and the Demon King Made Me a Minister (ヒラ役人やって1500年、魔王の力で大臣にされちゃいました, Hira Yakunin Yatte 1500-nen, Maō no Chikara de Daijin ni Sarechaimashita), published by SB Creative on the GA Novel imprint with Kisetsu Morita and Benio as the author and illustrator, respectively.

Red Dragon Girls' Secondary School) a special story about the character Laika was published at the end of those volumes.

A manga adaptation of the spin-off light novel I Was a Bottom-Tier Bureaucrat for 1,500 Years, and the Demon King Made Me a Minister was launched on Square Enix's Gangan Online website and smartphone app, with Meishi Murakami reprising his work.

An anime television series adaptation was announced during a livestream for the "GA Fes 2019" event on October 19, 2019.

During lunch at a cafe, Azusa gets inspiration from observing the strengths of others to run the coffee shop "The Witch's House" on the eve of the festival.