Ajin (亜人, Demi-Human) are the main species of the series. They are very like humans except for the fact that they are immortal and can summon IBM's.

Seventeen years before the beginning of the series, the Ajin were first discovered in Africa, when a soldier fighting on the battlefields rose from the dead countless times. This earned him the name "Soldier of God (神の兵 Kami no Hei)", and he soon became famous throughout the country he was fighting in. When word got out, an American government organization captured him and confirmed his immortality. While they tried to suppress this information from the public, it was simply too big, and it quickly became public knowledge.

There are no known method to discern Ajins from humans apart from them dying. However, it's possible for one Ajin to discover another Ajin if that person sees IBM. It is also possible for Ajins to make their Black Ghost visible to humans when Ajins feel extreme emotions such as rage and bloodlust.

The defining trait of all Ajin is their immortality. They are unable to die under any circumstances, such as being completely crushed, decapitated, or even due to malnutrition. The exact mechanism of this is unknown but Ikuya Ogura suggested that the only way it is possible would be because they possess an unknown matter in their body which replaces any body tissues or nutrient lost. If an Ajin is decapitated, their head will die and their body will regenerate a new head. It is currently unknown whether the new head would exhibit a duplicate personality, but the original personality of the Ajin would die, although the Ajin as an organism would continue to live. The regeneration can take from seconds to minutes depending on how severe the injury is and how much control the Ajin has over his/her IBM although committing fatal injuries (such as self-inflicted headshot) would allow them to regenerate quicker - this is known as "resetting" as Nagai calls it.

Ajin can also use a paralyzing scream that seizes the nervous system, paralyzing everyone in their immediate vicinity (its range is still unknown). Researchers think that this is a type of thanatosis reflex, which is why it works better on people who don't know about Ajin. Its effects on the affected person can also be neutralized by the user if he/she wishes to (as in the case of Nagai and Kaito). However, this is easily countered by wearing ear plugs or other noise dampeners. Ajins that are captured by the government have their mouth covered to prevent this from being used.

Theorised by Ogura to be the main cause of the Ajin's powers, all Ajin shown so far possess Invisible Black Matter (インビジブルブラックマター Inbijiburu Burakku Matā) which cannot be seen by normal humans. This matter is what allows the Ajin to replace any lost body tissues or nutrients, but there is a more lethal way in which it can be used. Created using the IBM, a Black Ghost (黒い幽霊 Kuroi Yūrei) is an entity which is manifested by an Ajin and most commonly used for combat purposes. However, it should be noted that not all Ajins are able to materialize a Black Ghost (Nakano being one). There are also no known method how an Ajin can properly materialize a Black Ghost although most of the characters in the series managed to manifest their Ghosts.