Cosmos 21 is the NASA designation for a Soviet spacecraft with unknown mission. This mission has been tentatively identified by NASA as a technology test of the Venera series space probes. It may have been an attempted Venus flyby, presumably similar to the later Cosmos 27 mission, or it may have been intended from the beginning to remain in geocentric orbit. In any case, the spacecraft never left Earth orbit after insertion by the SL-6/A-2-e launcher. The orbit decayed on November 14, three days after launch.

Beginning in 1963, the name Cosmos was given to Soviet spacecraft which remained in Earth orbit, regardless of whether that was their intended final destination. The designation of this mission as an intended planetary probe is based on evidence from Soviet and non-Soviet sources and historical documents. Typically Soviet planetary missions were initially put into an Earth parking orbit as a launch platform with a rocket engine and attached probe. The probes were then launched toward their targets with an engine burn with a duration of roughly 4 minutes. If the engine misfired or the burn was not completed, the probes would be left in Earth orbit and given a Cosmos designation.