"Zero Hour" is the twenty-fourth and final episode of the third season of the American science fiction television series Star Trek: Enterprise; the seventy-seventh episode overall. It first aired on May 26, 2004, on the UPN network within the United States. Set in the 22nd century, the series follows the adventures of the first Starfleet starship, Enterprise, registration NX-01. Season three of Enterprise features an ongoing story following an attack on Earth by previously unknown aliens called the Xindi.

The third season of the American science fiction television series Star Trek: Enterprise commenced airing on UPN in the United States on September 10, 2003 and concluded on May 26, 2004 after 24 episodes. Set in the 22nd century, the series follows the adventures of the first Starfleet starship Enterprise, registration NX-01, and its crew.