The Federation-Tzenkethi border was the division between the United Federation of Planets and the Tzenkethi. It was located in the Alpha Quadrant. The Federation had several colonies near it in the early 2370s, such as Barisa Prime. The Helaspont Nebula is also located nearby. On the Federation side of the border, there were gravitic sensor nets and subspace listening posts.
In late 2371, a Founder took control of the USS Defiant and had the starship cross the border into Tzenkethi space. (TNG: "Face of the Enemy"; DS9: "The Adversary")