The prostate is a small gland in men. It is part of the male reproductive system. The prostate is about the size and shape of a walnut. It sits low in the pelvis, below the bladder and just in front of the rectum. The prostate helps make semen, the milky fluid that carries sperm from the testicles through the penis when a man ejaculates. The prostate surrounds part of the urethra, a tube that carries urine out of the bladder and through the penis.