A sex organ, or primary sexual characteristic, as narrowly defined, is any of the anatomical parts of the body which are involved in sexual reproduction and constitute the reproductive system in a complex organism; flowers are the reproductive organs of flowering plants,cones are the reproductive organs of coniferous plants, whereas mosses, ferns, and other similar plants have gametangia for reproductive organs.

The visible portion of the human genitals for males consists of scrotum and a penis, for females it consists of the labia, clitoris and vagina. Women have two genital orifices, the vagina and urethra, while men have only one, the urethra. Both male and female genitals have many nerve endings, resulting in pleasurable and highly sensitive touch.

In several human societies, particularly in conservative ones, genitals are considered erogenous zones and are considered a public indecency and sometimes even illegal if left uncovered in public.

In mammals, sex organs include:

In typical prenatal development, sexual organs originate from a common anlage anatomy during early gestation and differentiate into male or female variations. 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.

In biology, sexual reproduction is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety, each known as a sex. Sexual reproduction involves combining specialized cells (gametes) to form offspring that inherit traits from both parents. Gametes can be identical in form and function (known as isogametes), but in many cases an asymmetry has evolved such that two sex-specific types of gametes (heterogametes) exist: male gametes are small, motile, and optimized to transport their genetic information over a distance, while female gametes are large, non-motile and contain the nutrients necessary for the early development of the young organism.

An organism's sex is defined by the gametes it produces: males produce male gametes (spermatozoa, or sperm) while females produce female gametes (ova, or egg cells); individual organisms which produce both male and female gametes are termed hermaphroditic. Frequently, physical differences are associated with the different sexes of an organism; these sexual dimorphisms can reflect the different reproductive pressures the sexes experience.

Organ may refer to the following: