Sector 21503 was a sector in Federation space near the Cardassian border, and near sector 21505 and the Cardassian sector.
During the Cardassian War, while in this sector, the USS Stargazer made an unsuccessful attempt to offer a truce to the Cardassian Empire.
In 2367, while on a mapping survey mission, the USS Enterprise-D was attacked by a Cardassian Galor-class ship in this sector because of earlier attacks by the USS Phoenix. (TNG: "The Wounded")