Kaiju No. 8 (Japanese: 怪獣8号, Hepburn: Kaijū Hachigō), also known in English as Monster #8, is a Japanese manga series written and illustrated by Naoya Matsumoto [ja]. It has been serialized on Shueisha’s Shōnen Jump+ app and website since July 2020, with its chapters collected in 14 tankōbon volumes as of November 2024. Shueisha has also published the series digitally on its Manga Plus service in English and several other languages. Viz Media began publishing the series digitally in August 2020 and has published the series in print in North America since December 2021.

After the destruction of their hometown, childhood friends Kafka Hibino and Mina Ashiro make a pact to become officers in the Defense Force—a militarized organization tasked with protecting Japan from colossal monsters known as “kaijuu.” Decades later, the 32-year-old Kafka has all but given up on his dreams of heroism. Instead, he cleans up the remains of the slaughtered kaijuu after they are defeated by valiant soldiers—including Mina, who has successfully achieved their shared goal.

Upon meeting his new coworker, Reno Ichikawa, Kafka faces a mirror of his past self: an ambitious young man whose one desire is to fight as a member of the Defense Force. Unfortunately, the two are soon involved in a freak encounter with a rogue kaijuu. Though Kafka demonstrates his innate heroic nature and rescues Reno from certain doom, he is left gravely injured.

While both men recover in a hospital, Kafka is seemingly attacked by another one of the beasts. As a result, he gains the ability to transform into a humanoid kaijuu with the strength and powers of the massive monsters menacing Japan. Dubbed “Kaijuu No. 8” by the military, Kafka resolves to use his newfound gifts for the greater good. Tied together by mutual respect, Kafka and Reno set out to join warriors like Mina at the forefront of the Defense Force.