The T'lli Beta system was a star system and the location of the planet T'lli Beta, home of the T'lli Betans.
In 2367, the Enterprise-D was en route to the T'lli Beta system 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")
The T'lli Beta system later appeared in the starchart Data and Picard were studying in Stellar Cartography in 2371, where it was located near the Lima Sierra system. (Star Trek Generations)