A gonad, also known as a sex gland or reproductive gland, is a mixed gland that produces the gametes and sex hormones of an organism. The gonads are the primary reproductive organs responsible for producing the sperm and ova, but they also secrete hormones and are considered to be endocrine glands. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad is the testicle, which produces sperm in the form of spermatozoa. The female gonad is the ovary, which produces egg cells. Both of these gametes are haploid cells. Some hermaphroditic animals and humans have a type of gonad called an ovotestis.

The development of the gonads is a part of the development of the urinary and reproductive organs. Gonads start developing as a common primordium, in the form of genital ridges, which are only later differentiated to male or female sex organs. The presence of the SRY gene, located on the Y chromosome and encoding the testis determining factor, usually determines male sexual differentiation. In the absence of the SRY gene from the Y chromosome, usually, the female sex (ovaries instead of testes) will develop.

The gonads are subject to many diseases, such as hypergonadism, hypogonadism, agonadism, tumors, and cancer, among others. Gonadal disorders can also contribute to infertility in both males and females. Hormone imbalance due to gonad disorders may be treated with hormone medications.