A hellmouth, or the jaws of Hell, is the entrance to Hell envisaged as the gaping mouth of a huge monster, a subject that first appears in Anglo-Saxon art. Hellmouths are frequently included in scenes of the Last Judgment and Harrowing of Hell through the end of the Middle Ages in Europe and sometimes, during the Renaissance and later. Hellmouths appeared in polemical popular prints after the Protestant Reformation, when figures from the opposite side would be shown disappearing into the mouth. A notable late appearance is in the two versions of a painting by El Greco of about 1578. Political cartoons showed Napoleon leading his troops into one.

Medieval theatre often included a hellmouth prop or mechanical device that was used to attempt to scare the audience by vividly dramatizing an entrance to Hell. These seem often to have featured a battlemented castle entrance, in painting usually associated with Heaven.

A hellmouth was intended to remind a Christian audience of the danger of damnation. Those shown entering, or already inside, are typically shown naked, their clothing not having survived the general resurrection of the dead that is often part of the same image. Some, even if naked, wear headgear indicating their rank at the top of society, with the papal tiara, king's crown and bishop's mitre the most common. Far rarer are indications of people being non-Christian, such as the Jewish hat.

According to art historian Meyer Schapiro, the oldest example of an animal-like hellmouth appears on an an ivory carving of ca. 800 in the Victoria and Albert Museum. Schapiro notes most examples before the 12th century are English with many showing the Harrowing of Hell, which appealed to Anglo-Saxon taste, as a successful military raid by Christ. Schapiro has speculated that the subject may have drawn from the pagan myth of the Crack of Doom, with the mouth that of the wolf-monster Fenrir, slain by Vidar, who is used as a symbol of Christ on the Gosforth Cross and other pieces of Anglo-Scandinavian art. In the assimilation of Christianised Viking populations in northern England, pagan mythological imagery merged Christian ones, in hogback grave markers for example.

Satan himself is often shown sitting in Hell eating the damned. According to G. D. Schmidt this is a separate image, and a hellmouth should not be considered to be the mouth of Satan, although Hofmann is inclined to disagree with this.

In the Anglo-Saxon Vercelli Homilies (4:46–48) Satan is likened to a dragon swallowing the damned:

... ne cumaþ þa næfre of þæra wyrma seaðe & of þæs dracan ceolan þe is Satan nemned.

[they] never come out of the pit of snakes and of the throat of the dragon which is called Satan.