A power bordering the Federation which fought a war against it in the mid-24th century, and which the Dominion tried to provoke into a fresh conflict in 2371.
The Tzenkethi are a spacefaring power led by a ruler called the Autarch. They are never shown on screen, but are spoken of as former enemies of the Federation. Benjamin Sisko fought Tzenkethi raiders aboard the USS Okinawa under Captain Leyton, as recalled in The Adversary and Paradise Lost.
In The Adversary, a Changeling posing as Ambassador Krajensky tells Sisko that the Autarch has been overthrown and sends the Defiant to the border. The coup is an invention. The infiltrator seizes the ship and sets it on course to attack a Tzenkethi settlement on M'kemas III, hoping to start a war, and is stopped only at the last moment.
The Tzenkethi still hold their own territory in the 32nd century. In the Discovery episode Jinaal, a clue to the Progenitors' technology is found to lie inside Tzenkethi space.