Which episode introduced the Klingons to Star Trek?

The Star Trek episode that introduced the Klingons was “Errand of Mercy.”

The episode is the 26th installment of The Original Series’ first season and first aired on March 23, 1967. Its story sends the USS Enterprise to Organia, a strategically important world near the Klingon border, after the Federation and Klingon Empire enter open conflict. Captain Kirk and Spock encounter the Klingon occupation led by Commander Kor, played by John Colicos.

“Errand of Mercy” established the Klingons as a major adversarial power in Star Trek. The Organians, who appear peaceful and technologically primitive, ultimately reveal overwhelming abilities and forcibly end the conflict, imposing a peace treaty on both sides. That ending sets up the long-running Federation–Klingon relationship seen throughout later series and films.

Viewers sometimes confuse this episode with “The Trouble with Tribbles,” which features Klingon political hostility, or “The Enterprise Incident,” which centers on the Romulans. “The Enterprise Incident” is also from the original series, but it does not introduce the Klingons.