"Elaan of Troyius" is a third season episode of the American science fiction television series, Star Trek, and was broadcast December 20, 1968. It is episode #68, production #57, and was written and directed by John Meredyth Lucas, the only person in Star Trek production history ever to both write and direct a filmed Star Trek television episode, whether animated or live-action.

In the episode, the USS Enterprise ferries a spoiled Elasian princess whose betrothal to a Troyian royal is hoped will bring peace to a star system at war. The episode's title character pays tribute to Helen of Troy from Homer's Iliad, and its plot borrows from William Shakespeare's comedy, The Taming of the Shrew.

On stardate 4372.5, the Federation starship USS Enterprise arrives at the planet Troyius, the outermost world in the Tellun system, to pick up Ambassador Petri, a green-skinned, white-haired Troyian. He is then taken to Elas, the innermost planet, to pick up the Dohlman, Elaan – an exotic, black-haired, beautiful but very demanding young woman who is a member of the Elasian royal family. The rulers of Elas and Troyius (the Council of Nobles and the Troyius Tribunal respectively) have mutually agreed to marry Elaan to a royal of Troyius (presumably the King, as Petri refers to him as "our ruler"), to secure peace before the two planets destroy each other.

The Tellun system lies in a border area between the Federation and the Klingon Empire. The Federation hopes this interplanetary royal marriage will bring peace and sway the Troyians and the Elasians toward the Federation rather than the Empire. Elaan is a most reluctant bride, cursing the arrangement that is, as she sees it, sending her off to be married to an enemy. Ambassador Petri's mission is to make the spoiled and arrogant Elaan acquainted with the manners and customs of the Troyians whose Queen she will be.

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