Jashin (ジャシン, Literally meaning: wicked heart; evil god; evil design) is the god worshiped in Jashinism (ジャシン教, Jashinkyō). Jashin's symbol is an upside down triangle with a circle around it.
Little of Jashin and his ways is known to those who aren't his followers, so named "Jashinists". According to Hidan, Jashin demands nothing less than utter death and destruction. In Akatsuki Hiden, Hidan frames this as a public service, saving people from the fear of death by killing them. One's neighbours are the most ideal target, but Hidan explains to Ameyuki that a "neighbour" requires more than physical proximity; Hidan spends most of his time with Kakuzu, but he will never consider Kakuzu to be his neighbour because he hates Kakuzu. Hidan always prays to Jashin before a fight, asking for a "good kill". If he is unsuccessful, or is not allowed to kill, he prays for forgiveness.
In Konoha Shinden, Konoha investigated Jashinism following their encounter with Hidan. They were unable to turn up much conclusive information about the religion, and in fact could not locate a single worshipper. With so little information to go on, Konoha's official report offered the possibility that Jashin was in fact an invention of Hidan. Several years after the Fourth Shinobi World War, Konoha joins forces with Yugakure to investigate Jashinism again. In doing so, they uncover a sect of Jashinists led by Ryūki, who have been kidnapping travellers to sacrifice to Jashin. Mirai Sarutobi, with help from Kakashi Hatake and Might Guy, is able to stop the Jashinists and save the sacrifices.
Hidan is the "successful experiment of the Jashin religion's secret techniques", which makes him immortal; lack of nutrition is one of the known methods that can lead to said immortal's death. Ryūki indicates this immortality is granted by Jashin in exchange for the ritualistic sacrifice of several people at once; Ryūki exclusively gathers young girls for this purpose, but it's unclear if this is necessary for the ritual. In addition to immortality, Jashin's followers can use certain jujutsu, creating voodoo-like links with targets whose blood they've ingested for as long as they remain within the perimeter of Jashin's symbol. With Hidan's Death Controlling Possessed Blood, he links himself to his targets, causing them to experience any injuries that he does; because he's immortal, he can kill targets by giving himself fatal wounds. With Ryūki's Body Controlling Manipulated Blood, he links targets to a puppet, causing anything that happens to the puppet to happen to them and forcing them to mimic any movement the puppet makes, regardless of whether they're alive.
- The upside-down triangle symbol of the Jashinism resembles the reversed pentagram, which was used by Western cults as a means to symbolise evil and attract evil forces.