From Latin Mārtius (“of or relating to the planet Mars”) + -an (adjective-forming suffix). The word is cognate with Middle English marcien, marcyan, mercien (“subject to the influence or power of the planet Mars; relating to the god Mars, that is, warlike”), Middle French martien (“Martian”) (modern French martien), French Martien (“imaginary inhabitant of Mars; any extraterrestrial”), Italian marziano, Latin Mārtiānus (“Martian”), Portuguese marciano, Spanish marciano.

Etymology: Wiktionary (CC-BY-SA)