"Villain of the week" (or, depending on genre, "monster of the week" or "freak of the week") is a term that describes the nature of one-use antagonists in episodic fiction, specifically ongoing American genre-based television series. As many shows of this type air episodes weekly at a rate of one or two dozen new episodes per year, there is often a new antagonist to forward the plot of each week's episode. The main characters usually confront and vanquish these characters, often leaving them never to be seen again. Some series alternate between using such antagonists and furthering the series' ongoing plotlines (as in The X-Files, where fandom is often divided over preference for one type of episode versus the other). Others use these one-time foes as pawns of the recurring adversaries (as in Power Rangers). On other occasions, these villains return reformed becoming invaluable allies.

Sometimes the villain will return later in the series but now having a larger role in the story.