A DemoSat (Demonstration Satellite) is a nonfunctional satellite used to test a carrier rocket, typically on its maiden flight. The idea is to provide a payload similar in mass and other characteristics to a functioning satelllite without risking the loss of a real spacecraft. As well as testing new rockets, DemoSats may be used to check out launch systems upon return-to-flight after launch failures.

Defunct satellites from cancelled programs may be flown as DemoSats, for example the maiden flight of the Soyuz-2 rocket placed an obsolete Zenit-8 satellite onto a sub-orbital trajectory in order to test the rocket's performance.