An estimated 25%-40% of infertile men have idiopathic infertility associated with deficient sperm numbers and quality. Here, we identify the membrane-anchored serine protease PRSS21, also known as testisin, to be a novel proteolytic factor that directs epididymal sperm cell maturation and sperm-fertilizing ability. PRSS21-deficient spermatozoa show decreased motility, angulated and curled tails, fragile necks, and dramatically increased susceptibility to decapitation. These defects reflect aberrant maturation during passage through the epididymis, because histological and electron microscopic structural analyses showed an increased tendency for curled and detached tails as spermatozoa transit from the corpus to the cauda epididymis. Cauda epididymal spermatozoa deficient in PRSS21 fail to mount a swelling response when exposed to hypotonic conditions, suggesting an impaired ability to respond to osmotic challenges facing maturing spermatozoa in the female reproductive tract. These data suggest that aberrant regulation of PRSS21 may underlie certain secondary male infertility syndromes, such as "easily decapitated" spermatozoa in humans.