A sex organ, also known as a reproductive organ, is a part of an organism that is involved in sexual reproduction. Sex organs constitute the primary sex characteristics of an organism. Sex organs are responsible for producing and transporting gametes, as well as facilitating fertilization and supporting the development and birth of offspring. Sex organs are found in many species of animals and plants, with their features varying depending on the species.

Sex organs are typically differentiated into male and female types. In humans, the male sex organs include the testes, penis, and prostate gland; the female sex organs include the ovaries, fallopian tubes, uterus, cervix, vagina, and vulva.

In animals (including humans), the testis in the male and the ovary in the female are called the primary sex organs. All other sex-related organs are called secondary sex organs, divided between internal and external sex organs. External sex organs include the genitals or external genitalia, visible at birth in both sexes.

In plants, male reproductive structures include stamens in flowering plants, which produce pollen. Female reproductive structures, such as pistils in flowering plants, produce ovules and receive pollen for fertilization. Mosses, ferns, and some similar plants have gametangia for reproductive organs, which are part of the gametophyte. The flowers of flowering plants produce pollen and egg cells, but the sex organs themselves are inside the gametophytes within the pollen and the ovule. Coniferous plants likewise produce their sexually reproductive structures within the gametophytes contained within the cones and pollen. The cones and pollen are not themselves sexual organs.

Together, the sex organs constitute an organism's reproductive system.