Seventeen years ago, mysterious immortal soldiers known as divine warriors appeared on the battlefields of Africa. These life forms later became known as Ajin, or demi-humans, and were considered to be one of the greatest discoveries of mankind. The government declared their existence to be compatible with humans; however, it is rumored that they instead offer bounties for captured Ajin to secretly perform inhumane experiments on these rare beings.

Kei Nagai, an apathetic high school student, is studying to become a doctor and only wishes to live a normal life. However, this simple dream is shattered after he suddenly gets into a deadly traffic accident. Revived at the scene, he is immediately labeled as an Ajin by bounty-hungry witnesses. Unable to understand his immortality or prove his innocence, he starts to live his life on the run and fights for survival. He barely escapes the government's reach when a group of anarchistic Ajin contacts Kei, asking him to join their forces to retaliate against the government. But who is Kei's real enemy, and which side will he take when it comes down to humans versus Ajin?

In 2013, Sakurai Gamon stated on his blog that he would continue working on Ajin alone from volume two onward. The reason for co-author Miura Tsuina's withdrawal was not given.

Ajin was published in English as Ajin: Demi-Human by Vertical Inc. from October 14, 2014, to December 14, 2021, and as omnibus volumes since October 29, 2024. The series was published in Spanish by Norma Editorial from October 29, 2015, to May 27, 2022; in German by Egmont Manga from June 5, 2015, to July 1, 2021; in Polish by Studio JG from April 5, 2016, to January 17, 2022; in Brazilian Portuguese by Panini Comics from February 25, 2016, to September 10, 2021; in Italian by Edizioni Star Comics from May 9, 2015, to November 10, 2021; and in Russian by XL Media from October 2017 to May 2023.