"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.

In this episode, Sub-Commander T'Pol leads the Enterprise on an attack on 'Sphere 41' in an attempt to destroy the sphere network within the Delphic Expanse. Meanwhile, a team led by Captain Jonathan Archer has infiltrated the Xindi weapon while it is en route to Earth. Both the spheres and the weapon are destroyed, but Archer is lost, presumed dead. Enterprise returns to Earth but the crew suddenly find that they have been sent back in time to a very different Second World War.

The episode was written by series co-creators Rick Berman and Brannon Braga and was directed by Allan Kroeker. The episode took eight days to film and included a larger guest cast than normal for episodes of Enterprise, as well as two competition winners. The twist at the end of "Zero Hour" was originally conceived by the writers as a joke, but was developed over the course of the third season to become the concept of "alien Nazis". This twist was further explained and resolved in the season four two-part opener "Storm Front". Roughly 3.91 million viewers watched "Zero Hour" on first broadcast.