The Breen system was an inhabited planetary system in the Alpha Quadrant. A planet in this system, Breen, was the homeworld of the Breen, a warp-capable humanoid species. This was the capital system of the Breen Confederacy and, in 2375, was allied with the Dominion. (DS9: "For the Uniform")

This system was only mentioned in dialogue.
According to Star Trek: Star Charts (p. 36) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the Breen system's primary was a M-class star.