Sector 21503 was a region of space. This sector was adjacent to the Cardassian sector and located near Sector 21505. The Federation-Cardassian border was located in this sector.

During the Federation-Cardassian War, while in this sector, the USS Stargazer was sent to make preliminary overtures to a truce. When within range of a Cardassian warship, Captain Picard ordered the shields lowered as a measure of good faith. The warship open fired onto the Stargazer, taking out most of the weapons and damaging the impulse engines of the Federation starship. The Stargazer retreated from the sector with the warship in pursuit.

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")

This sector was only mentioned in dialogue.