T'lli Beta was an inhabited planet in the T'lli Beta system. This was the homeworld of the T'lli Betans.

In 2367, the USS 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 lifeforms were attempting to enter. (TNG: "The Loss")

In 2371, the location of T'lli Beta was labeled in the star chart Data and Picard were studying in stellar cartography aboard the USS Enterprise-D. (Star Trek Generations, okudagram)

According to Star Trek Encyclopedia (3rd ed., p. 489), this planet was named after writer Hilary J. Bader's grandmother, Tillie Bader.