The development of different sexes has fascinated humankind for millennia. In plants unisexual flowers evolved multiple times independently, yet the molecular mechanisms governing sex determination remain largely elusive. Here, we analysed sex determination in Cannabis sativa, one of the world\'s oldest and most versatile crops. C. sativa is primarily dioecious, with separate male and female individuals, but some cultivars are monoecious, bearing both male and female flowers on the same plant. Using QTL mapping we identified Monoecy1, a locus on the X chromosome controlling monoecy vs dioecy. Strikingly, transcriptomic and genomic analyses revealed that Monoecy1 contains three genes within just 60,000 bp which have distinct male-, female- and monoecy-specific expression patterns and encode for putative developmental regulators. We propose that the combinatorial interaction of those genes can explain not only the molecular control of monoecy vs. dioecy but also of male vs. female individuals.