Monoecy (from Ancient Greek μονοικία (monoikía) 'one household'; adj. monoecious), is a sexual system in seed plants where separate male and female cones or flowers are present on the same plant. It is a monomorphic sexual system comparable with gynomonoecy, andromonoecy and trimonoecy, and contrasted with dioecy where individual plants produce cones or flowers of only one sex, and with bisexual or hermaphroditic plants in which male and female gametes are produced in the same flower.

Monoecy often co-occurs with anemophily, because it prevents self-pollination of individual flowers and reduces the probability of self-pollination between male and female flowers on the same plant.

Monoecy in angiosperms has been of interest for evolutionary biologists since Charles Darwin.