A perimeter alert was an alert used on Starfleet vessels, to warn the crew of an approaching, unidentified starship.

In 2364, Wesley Crusher, while sitting in the command chair on the USS Enterprise-D, flipped open a control panel and told Captain Jean-Luc Picard of a perimeter alert when an alien entity approached Deneb IV. (TNG: "Encounter at Farpoint")

In 2372, Tom Paris announced a perimeter alert when the Vidiians approached the USS Voyager. (VOY: "Deadlock")