The prostate gland is part of the male reproductive system. It is a small, muscular organ about the size of a walnut, located at the base of the bladder and in front of the rectum.

The prostate, together with the seminal vesicles and the Cowper's glands, comprise the male sexual accessory glands. They produce over 95% of the ejaculate by means of which the sperm is transported into the female reproductive tract.

Specific compounds in the secretions of each gland contribute to fertilization, for example by maintaining the optimal fluid viscosity, encouraging the viability of the sperm cells, or insulating them from the female immune system. The prostate itself produces between about 15 and 30% of the ejaculate. Its secretions include citric acid, which maintains the osmotic equilibrium between the fluid and the sperm cells so that they do not become either engorged or dehydrated. It also contains zinc, a bactericide that may help prevent urinary tract infections.

Sperm cells from the testes plus the fluid from the seminal vesicles enter the prostate via a tube called the vas deferens. In the prostate, the additional secretions are added, and the muscular gland pumps the fluid into the urethra during ejaculation.

The prostate surrounds the urethra like a collar, so a swollen prostate is a common cause of urinary problems in men, especially as they age. About one in four will eventually require surgery for benign prostate hyperplasia, and prostate cancer is now the most common malignancy in men. Early diagnosis of prostate cancer is now possible by means of a blood test for prostate-specific antigen (PSA). PSA is a protein that is normally found in the ejaculate, and enters the blood when the prostate is damaged.