Tesnia was the homeworld of the Tesnians.

In 2151, Enterprise NX-01 rescued a group of Tesnians from their critically-damaged ship and transported them back to Tesnia. (ENT: "Shuttlepod One")

This planet was only mentioned in dialogue.
According to Star Trek: Star Charts (p. 60) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), there was a system named Tesnia, which might possibly be the location of this planet, in the Alpha Quadrant. The system's primary was a F-class star. In the late 24th century, this was an independent system.