The Greek god Thanatos (死を司る神タナトス, Shi o tsukasadoru kami Tanatosu?, shi o tsukasadoru kami meaning "death-ruling god") is one of the antagonist deities featured in the universe of the Manga Saint Seiya, authored by Masami Kurumada, that was later adapted to Anime. He was created by Kurumada based on the mythologic persona of the same name.

As in Greek mythology, he is the personification of mortality and death, and the twin brother of Hypnos, the Greek god of sleep. In Kurumada's mythos, although he is a subordinate of Hades, the Emperor of the Darkness and ruler of the Underworld, he is a deity with all attributes, and as his lord, he is a sworn enemy of the protector goddess of the Earth, Athena.

Kurumada introduced Thanatos in the final parts of the Hades arc of his manga, specifically in vol. 27, and he is portrayed as very cruel, merciless, hateful towards humans, haughty, and non-hesitant to whimsically inflict death. He was sealed along with his brother by Athena after being defeated in her last battle against Hades, in 18th century. Thanatos spent 243 years sealed in a box marked with the Seal of Athena, and only regained his freedom after the box was opened by a curiosity-driven 3 year-old Pandora, to whom he announced that the resurrection of Hades was near, and that she would serve them henceforth.

As described by the Pegasus Saint Seiya when they first encountered, he is identical to his brother Hypnos, the only differences being Thanatos' silver colored eyes and hair, as opposed to Hypnos' golden. A strange looking star is placed in his forehead. Unlike his brother Hypnos, who is more calm and calculating, Thanatos is prone to lose his temper and act without any regard for consequences when influenced by rage.

His dwelling place is the Elysion, and he spends most of his time there accompanied by scores of Nymphs, and his twin brother Hypnos, waiting to fulfill their lord Hades's bidding. Thanatos offered fierce opposition to the Saints of Athena, when they tried to rescue their goddess from certain death and stop Hades's Greatest Eclipse, which was meant to eradicate mankind.

Thanatos duty is to impart death on all living beings when their time has come, or as divine retribution. Before his formal introduction in the anime adaptation, he was featured in the opening of the Inferno stage of the Hades arc episodes (episode 128 and on), standing next to his brother Hypnos, and overshadowed by Hades. His full appearance in the anime adaptation occurred in 2008, in the final episodes that adapt the last two volumes of the manga, in which his participation is contained (vols. 27 and 28), 16 years after Kurumada introduced him in the manga. From episode 142 and on, his silhouette in the opening of the anime adaptation revealed his features fully.