What episode features the first appearance of Romulans in Star Trek?

The episode featuring the first appearance of Romulans in Star Trek is “Balance of Terror.”

In this first-season episode, the USS Enterprise investigates attacks on Federation outposts along the Romulan Neutral Zone. Because the earlier Earth-Romulan War was fought without visual communication, neither side knows what the other looks like.

The Enterprise discovers that the attackers are Romulans, whose pointed ears and appearance reveal a connection to Spock’s Vulcan ancestry. The episode also introduces the Romulan cloaking device and establishes the Romulans as a militaristic rival of the Federation.

“Balance of Terror” first aired on December 15, 1966. Its central conflict is a tense cat-and-mouse battle between Kirk’s Enterprise and a Romulan vessel commanded by Mark Lenard’s unnamed Romulan Commander. Lenard later became famous in Star Trek as Sarek, Spock’s father. A common mix-up is “The Enterprise Incident,” which also features Romulans and their technology, but it occurs later in the third season and is not their debut.