Mithren was an inhabited world in the Delta Quadrant, located near Bothan territory.

Neelix had some contacts on Mithren, who informed him in 2372 that they had lost a number of vessels to the Botha. Shortly thereafter, after Voyager believed they had captured a Botha, they suggested to him that they might turn him over to the government of Mithren. (VOY: "Persistence of Vision")

Voyager remained in contact with Mithren for some time, contacting it at least two times in a two-month period leading up to stardate 49485. (VOY: "Investigations")

This planet was only mentioned in dialogue.
According to Star Trek: Star Charts (p. 82), there was a star system named Mithren, in which this planet might possibly have been located, in the quadrant. The system's primary was a M-class star.