Sector 21527 was a region of space. This sector contained an unspecified asteroid field.

In the mid-24th century, the location of the Cardassian-Federation border running through this sector, and the adjacent sector, was in dispute. The Cardassians believed the placement of this border was the result of a miscalibration error by a reference beacon. According to them, the border should have been 4.7 light years closer to the galactic center. (TNG-R: "Chain of Command, Part II")

In 2370, the USS Enterprise-D performed tests of new weapons systems in this sector. Later, when the ship's crew was disabled by Barclay's Protomorphosis Syndrome, the Enterprise-D became adrift in this sector. (TNG: "Genesis")

For the pre-mastered "Chain of Command, Part II", a star chart was displayed on the bridge of the Enterprise which supposedly showed the location of the McAllister C-5 Nebula. For the remastered version, the map was changed. The most notable change was the sector number, which was changed from Sector 21527 to Sector 71527.