A hellhound is a supernatural dog in folklore. A wide variety of ominous or hellish supernatural dogs occur in mythologies around the world, similar to the oft-seendragon. Features that have been attributed to hellhounds include black fur, glowing red - or sometimes yellow - eyes, super strength or speed, ghostly or phantom characteristics, foul odor, and sometimes even the ability to talk. Certain European legends state that if someone stares into a hellhound's eyes three times or more, that person will surely die. In cultures that associate the afterlife with fire, hellhounds may have fire-based abilities and appearance. They are often assigned to guard the entrances to the world of the dead, such as graveyards and burial grounds, or undertake other duties related to the afterlife or the supernatural, such as hunting lost souls or guarding a supernatural treasure. In European legends, seeing a hellhound or hearing it howl may be an omen or even a cause of death.

The most famous hellhound is probably Cerberus from Greek mythology. Hellhounds are also famous for appearing in Northern European mythology and folklore as a part of the Wild Hunt. These hounds are given several different names in local folklore, but they display typical hellhound characteristics. The myth is common across Great Britain.

In southern Mexican and Central American folklore, the Cadejo is a big black dog that haunts travellers who walk late at night on rural roads. The term is also common in American blues music, such as in Robert Johnson's "Hellhound on my Trail"

In Greek mythology the hellhound belonged to Hades, the Greek god of death and the underworld, its name in Greek mythology is Cerberus, it has three heads but is still black with razor sharp teeth and super strength, it is used to guard the gates of hell.