Trioecy /traɪˈiːsi/, also spelled triecy, is a sexual system characterized by the coexistence of males, females, and hermaphrodites. It has been found in both plants and animals. Like androdioecy and gynodioecy, trioecy is a mixed mating systems.

Trioecy is also called tridioecy /ˌtraɪdaɪˈiːsi/ and subdioecy /ˌsʌbdaɪˈiːsi/.

The term trioecy comes from the Neo-Latin word Trioecia, a former order of trioecious plants.

Trioecy may be an unstable transient state associated with evolutionary transitioning from gynodioecy to dioecy. In brachiopod species, trioecy usually breaks into androdioecy or gynodioecy. Other studies show that trioecious populations originated from gonochoristic ancestors which were invaded by a mutant selfing hermaphrodite, creating a trioecious population. It has been suggested that chromosomal duplication plays an important part in the evolution of trioecy.

But one study found that trioecy can be stable under nucleocytoplasmic sex determination. Another theoretical analysis indicates that trioecy could be evolutionary stable in plant species if a large amount of pollinators vary geographically.

Trioecy is a relatively common sexual system in plants, estimated to occur in about 3.6% of flowering plant species, although most reports of trioecy could be misinterpretations of gynodioecy. It is rare as well as poorly understood in animals.

The following species have been observed to exhibit a trioecious breeding system.