Cygnus NG-21 is the twenty-first planned flight of the Cygnus robotic resupply spacecraft and its eighteenth flight to the International Space Station (ISS). It is planned to launch in August 2024. It is contracted to Northrop Grumman under the Commercial Resupply Services II (CRS-2) contract with NASA. The capsule is scheduled to be launched aboard a SpaceX Falcon 9 rocket.

Commercial Resupply Services (CRS) are a series of flights awarded by NASA for the delivery of cargo and supplies to the International Space Station (ISS) on commercially operated spacecraft. The first CRS contracts were signed in 2008 and awarded $1.6 billion to SpaceX for twelve cargo Dragon and $1.9 billion to Orbital Sciences for eight Cygnus flights, covering deliveries to 2016. The Falcon 9 and Antares rockets were also developed under the CRS program to deliver cargo spacecraft to the ISS.