In Greek mythology, the river Phlegethon ("[river of] fire") was one of the five rivers of the underworld. It flowed with fire that burned but did not consume fuel. In the Divine Comedy the river is made of boiling blood and is part of the seventh circle of hell, containing the shades of tyrants, murderers, robbers and those guilty of sins involving violence against others.

Virgil mentions Phlegethon with the other infernal rivers in the Aeneid, Book VI, 265, 551.