A sex organ (or reproductive organ) is any part of an animal's body that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. The testis in the male, and the ovary in the female, are called the primary sex organs.^[1] The external sex organs – the genitals or genitalia, visible at birth in both sexes, and the internal sex organs are called the secondary sex organs.^[1]

Mosses, ferns, and some similar plants have gametangia for reproductive organs, which are part of the gametophyte.^[2] 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.^[3] 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.

The Latin term genitalia, sometimes anglicized as genitals, is used to describe the externally visible sex organs: in male mammals, the penis and scrotum; and in female mammals, the vulva and its organs.

The genitals and the internal sex organs are referred to as the secondary sex organs. The primary sex organs are the gonads, a pair of sex organs, specifically the testes in the male or the ovaries in the female. Gonads as primary sex organs, generate reproductive gametes containing inheritable DNA. They also produce most of the primary hormones that affect sexual development, and regulate other sexual organs and sexually differentiated behaviors.

In general zoology, given the great variety in organs, physiologies, and behaviors involved in copulation, male genitalia are more strictly defined as "all male structures that are inserted in the female or that hold her near her gonopore during sperm transfer"; female genitalia are defined as "those parts of the female reproductive tract that make direct contact with male genitalia or male products (sperm, spermatophores) during or immediately after copulation".^[4]

The visible portion of the mammalian genitals for males consists of the scrotum and penis; for females, it consists of the vulva (labia, clitoris, etc.) and vagina.

In placental mammals, females have two genital orifices, the vagina and urethra, while males have only one, the urethra.^[5] Male and female genitals have many nerve endings, resulting in pleasurable and highly sensitive touch.^[6][7] In most human societies, particularly in conservative ones, exposure of the genitals is considered a public indecency.^[8]

In mammals, sex organs include:

In typical prenatal development, sex organs originate from a common primordium during early gestation and differentiate into male or female sexes. The SRY gene, usually located on the Y chromosome and encoding the testis determining factor, determines the direction of this differentiation. The absence of it allows the gonads to continue to develop into ovaries.

Thereafter, the development of the internal, and external reproductive organs is determined by hormones produced by certain fetal gonads (ovaries or testes) and the cells' response to them.

The initial appearance of the fetal genitalia looks basically feminine: a pair of "urogenital folds" with a small protuberance in the middle, and the urethra behind the protuberance. If the fetus has testes, and if the testes produce testosterone, and if the cells of the genitals respond to the testosterone, the outer urogenital folds swell and fuse in the midline to produce the scrotum; the protuberance grows larger and straighter to form the penis; the inner urogenital swellings grow, wrap around the penis, and fuse in the midline to form the penile urethra.

Each sex organ in one sex has a homologous counterpart in the other one. See a list of homologues of the human reproductive system. In a larger perspective, the whole process of sexual differentiation also includes development of secondary sexual characteristics such as patterns of pubic and facial hair and female breasts that emerge at puberty. Furthermore, differences in brain structure arise, affecting, but not absolutely determining, behavior.

Intersex is the development of genitalia somewhere between typical male and female genitalia. Once the child is born, the parents are faced with decisions that are often difficult to make, such as whether or not to modify the genitalia, assign the child as male or female, or leave the genitalia as is. Some parents allow their doctors to choose. If they do decide to modify the genitalia, they have approximately a 50% chance of getting genitalia that will match the child's gender identity. If they pick the wrong one, their child may begin to show symptoms of transsexualism, which can lead them to a life of discomfort until they are able to remedy the issue.^[9]

Because of the strong sexual selection affecting the structure and function of genitalia, they form an organ system that evolves rapidly.^[10][11][12] A great variety of genital form and function may therefore be found among animals.

In many other animals a single posterior orifice, called the cloaca, serves as the only opening for the reproductive, digestive, and urinary tracts (if present). All amphibians, birds, reptiles, some fish, and a few mammals (monotremes, tenrecs, golden moles, and marsupial moles) have this orifice, from which they excrete both urine and feces in addition to serving reproductive functions. Excretory systems with analogous purpose in certain invertebrates are also sometimes referred to as cloacae.

The organs concerned with insect mating and the deposition of eggs are known collectively as the external genitalia, although they may be largely internal; their components are very diverse in form.

The reproductive system of gastropods (slugs and snails) varies greatly from one group to another.

Planaria are flat worms widely used in biological research. There are sexual and asexual planaria. Sexual planaria are hermaphrodites, possessing both testicles and ovaries. Each planarian transports its excretion to the other planarian, giving and receiving sperm.

The life cycle of land plants involves alternation of generations between a sporophyte and a haploid gametophyte. The gametophyte produces sperm or egg cells by mitosis. The sporophyte produces spores by meiosis which in turn develop into gametophytes. Any sex organs that are produced by the plant will develop on the gametophyte. The seed plants, which include conifers and flowering plants have small gametophytes that develop inside the pollen grains (male) and the ovule (female).

Sexual reproduction in flowering plants involves the union of the male and female germ cells, sperm and egg cells respectively. Pollen is produced in stamens, and is carried to the pistil, which has the ovary at its base where fertilization can take place. Within each pollen grain is a male gametophyte which consists of only three cells. In most flowering plants the female gametophyte within the ovule consists of only seven cells. Thus there are no sex organs as such.