Sh 2-21 is an emission nebula in the constellation Sagittarius. It is part of the Sharpless Catalog assembled by Stewart Sharpless. It is associated with four infrared clusters, [DB2000] 7, [DB2000] 10, [DB2000] 11 and [DB2000] 12. Open cluster Collinder 351 is included as part of the nebula complex.