Dioecy (/daɪˈiːsi/ dy-EE-see; from Ancient Greek διοικία dioikía 'two households'; adj. dioecious, /daɪˈiːʃ(i)əs/ dy-EE-sh(ee-)əs) is a characteristic of certain species that have distinct unisexual individuals, each producing either male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproduction is biparental reproduction. Dioecy has costs, since only the female part of the population directly produces offspring. It is one method for excluding self-fertilization and promoting allogamy (outcrossing), and thus tends to reduce the expression of recessive deleterious mutations present in a population. Plants have several other methods of preventing self-fertilization including, for example, dichogamy, herkogamy, and self-incompatibility.

In zoology, dioecy means that an animal is either male or female, in which case the synonym gonochory is more often used.^{[page needed]} Most animal species are gonochoric, almost all vertebrate species are gonochoric, and all bird and mammal species are gonochoric. Dioecy may also describe colonies within an animal species, such as the colonies of Siphonophorae (Portuguese man-of-war), which may be either dioecious or monoecious.

Land plants (embryophytes) differ from animals in that their life cycle involves alternation of generations. In animals, typically an individual produces gametes of one kind, either sperm or egg cells. The gametes have half the number of chromosomes of the individual producing them, so are haploid. Without further dividing, a sperm and an egg cell fuse to form a zygote that develops into a new individual. In land plants, by contrast, one generation – the sporophyte generation – consists of individuals that produce haploid spores rather than haploid gametes. Spores do not fuse, but germinate by dividing repeatedly by mitosis to give rise to haploid multicellular individuals, the gametophytes, which produce gametes. A male gamete and a female gamete then fuse to produce a new diploid sporophyte.

In bryophytes (mosses, liverworts and hornworts), the gametophytes are fully independent plants. Seed plant gametophytes are dependent on the sporophyte and develop within the spores, a condition known as endospory. In flowering plants, the male gametophytes develop within pollen grains produced by the sporophyte's stamens, and the female gametophytes develop within ovules produced by the sporophyte's carpels.

The sporophyte generation of a seed plant is called "monoecious" when each sporophyte plant has both kinds of spore-producing organ but in separate flowers or cones. For example, a single flowering plant of a monoecious species has both functional stamens and carpels, in separate flowers.

The sporophyte generation of seed plants is called dioecious when each sporophyte plant has only one kind of spore-producing organ, all of whose spores give rise either to male gametophytes, which produce only male gametes (sperm), or to female gametophytes, which produce only female gametes (egg cells). For example, a single flowering plant sporophyte of a fully dioecious species like holly has either flowers with functional stamens producing pollen containing male gametes (staminate or 'male' flowers), or flowers with functional carpels producing female gametes (carpellate or 'female' flowers), but not both. There are other, more complex reproductive schemes such as gynodioecy and androdioecy.

Slightly different terms, dioicous and monoicous, may be used for the gametophyte generation of non-vascular plants, although dioecious and monoecious are also used. A dioicous gametophyte either produces only male gametes (sperm) or produces only female gametes (egg cells). About 60% of liverworts are dioicous.

Dioecy occurs in a wide variety of plant groups. Examples of dioecious plant species include ginkgos, willows, cannabis and African teak. As its specific name implies, the perennial stinging nettle Urtica dioica is dioecious, while the annual nettle Urtica urens is monoecious. Dioecious flora are predominant in tropical environments.