A gonad, sex gland, or reproductive gland[1] is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm.[2] The male gonad, the testicle, produces sperm in the form of spermatozoa. The female gonad, the ovary, produces egg cells. Both of these gametes are haploid cells. Some hermaphroditic animals have a type of gonad called an ovotestis.

It is hard to find a common origin for gonads, but gonads most likely evolved independently several times.[3]

The gonads are controlled by luteinizing hormone and follicle-stimulating hormone, produced and secreted by gonadotropes or gonadotrophins in the anterior pituitary gland.[4] This secretion is regulated by gonadotropin-releasing hormone produced in the hypothalamus.[5][6]

Gonads start developing as a common primordium (an organ in the earliest stage of development), in the form of gonadal ridges,[7] and only later are differentiated to male or female sex organs. The presence of the SRY gene,[8] located on the Y chromosome and encoding the testis determining factor, determines male sexual differentiation. In the absence of the SRY gene from the Y chromosome, the female sex (ovaries instead of testes) will develop. The development of the gonads is a part of the development of the urinary and reproductive organs.