Sector 21505 was a sector of space that was located within Cardassian territory. The Cuellar system is located within the sector.

In 2367, the rogue starship USS Phoenix assaulted several targets in this sector, including an outpost in the Cuellar system. The USS Enterprise-D entered this region by order of Starfleet to assist the Cardassians in locating the Phoenix. The Phoenix successfully destroyed two Cardassian ships before the Phoenix could be stopped by the Enterprise-D. (TNG: "The Wounded")