A security two alert was a type of alert condition that called for the locating and restraint of an individual believed to be about to do another harm.

In 2266, Captain James T. Kirk enacted a security two alert aboard the USS Enterprise, following the revelation that Kevin Riley had escaped from sickbay, taken a phaser from the weapons locker and gone in search of Anton Karidian with the intent to murder him. (TOS: "The Conscience of the King")

In the final revised draft of the script for "The Conscience of the King", this alert was instead referred to as a "Red Security alert".