XIII may refer to:
- 13 (number) or XIII in Roman numerals
- XIII (comics), a Belgian comic book series by Jean Van Hamme and William Vance
- XIII (video game), a video game based on the comic book series
- XIII (miniseries), a TV miniseries based on the comic book series and game of the same title
- XIII: The Series, a 2011 TV series based on the comic book series and miniseries
- XIII (album), an album by Mushroomhead
- Factor XIII, a clotting factor inside the coagulation cascade