T'lli Beta is a planet in the T'lli Beta system, home to the T'lli Betans.
In 2367, the Enterprise-D was en route to T'lli Beta before it was ensnared by the gravitational pull of a cosmic string fragment which a group of two-dimensional beings were attempting to enter. (TNG: "The Loss")
T'lli Beta also appeared in the starchart Data and Picard were studying in Stellar Cartography in 2371. (Star Trek Generations)