1944, apparently coined by Polish-born U.S. jurist Raphael Lemkin (1900-1959) in his work "Axis Rule in Occupied Europe" [p.19], in reference to Nazi extermination of Jews, literally "killing a tribe," from Greek genos "race, kind" (see genus) + -cide. The proper formation would be *genticide.
Earlier in a similar sense was populicide (1799), from French populicide, by 1792, a word from the Revolution. This was taken into German, e.g. Völkermeuchelnden "genocidal" (Heine), which was Englished 1893 as folk-murdering.