"Balance of Terror" is the fourteenth episode of the first season of the American science fiction television series Star Trek. Written by Paul Schneider and directed by Vincent McEveety, it first aired on December 15, 1966.

In the episode, the USS Enterprise battles a Romulan ship after investigating an unidentified assailant who methodically destroys the Federation's outposts at the Neutral Zone.

"Balance of Terror" introduces the cloaking device and the Romulans. Mark Lenard plays the Romulan commander; he later portrayed Spock's father Sarek in the majority of his later Star Trek appearances, as well as a Klingon captain in Star Trek: The Motion Picture. The episode also features the final appearance of yeoman Janice Rand (Grace Lee Whitney), who would not appear again until Star Trek: The Motion Picture.

The episode's events are explored within an alternate future in the Star Trek: Strange New Worlds (2022) episode "A Quality of Mercy".