Tau Ceti Prime was a prime planet orbiting the star Tau Ceti.

Vice Admiral Janeway died on Tau Ceti Prime, when he drowned under an ice cap, in 2358. (VOY: "Coda")

According to Star Trek: Star Charts (p. 44) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the Kaferia (Tau Ceti) system was located in the Alpha Quadrant. In the late 24th century, this was a Federation system, with Tau Ceti III listed as a member.
According to the Star Trek Encyclopedia (4th ed., vol. 2, p. 375), Tau Ceti Prime was the first planet in the Tau Ceti system.