Military science fiction is a subgenre of science fiction featuring the use of science fiction technology, mainly weapons, for military purposes and normally principal characters that are members of a military service involved in military activity; occurring sometimes in outer space or on a different planet or planets.

A detailed description of the conflict, the tactics and weapons used for it, and the role of a military service and the individual members of that service forms the basis for a typical work of military science fiction. The stories often use features of actual past or current Earth conflicts, with countries being replaced by planets or galaxies of similar characteristics, battleships replaced by space battleships and certain events changed so that the author can extrapolate what might have occurred.