An antagonist is a character, group of characters, or institution that represents the opposition against which the protagonist or protagonists must contend. In other words, an antagonist is a person or a group of people who oppose the main character(s).

In the classic style of stories wherein the action consists of a hero fighting a villain/enemy, the two can be regarded as protagonist and antagonist, respectively. Of course, some narratives cast the villain the protagonist role, with the opposing hero as the antagonist.

The antagonist may also represent a major threat or obstacle to the main character by their very existence, without necessarily deliberately targeting him or her.

Examples in both film and theatre include Sauron, the main antagonist in The Lord of the Rings, who constantly battles the series' protagonists, and Tybalt, an antagonist in Romeo and Juliet, who slays Mercutio and whose later death results in the exiling of the play's protagonist, Romeo. A convention of the antagonist in a story is that their moral choices are less savoury than those of the protagonist. This is often used by an author to create conflict within a story. However, this is merely a convention and the reversal of this can be seen in the character Macduff from Macbeth, who is arguably morally correct in his desire to fight the tyrant Macbeth.