Micropenis is a medical condition in which a man has an unusually small penis. Micropenis is defined statistically as a penis at least 2.5 standard deviations smaller than the mean penis length. Assuming a normal distribution of penis length, according to this definition 0.6% of men are born with micropenis. The condition is usually diagnosed shortly after birth. For newborn boys, micropenis is diagnosed when the stretched flaccid penis measures less than 1.9 cm from base to tip. Micropenis does not indicate any deficiency in the function of the male reproductive system, merely that the penis is small but otherwise functional.

Micropenis can have numerous causes and may indicate Klinefelter syndrome, but more commonly it simply indicates that either the mother or developing male fetus failed to produce, or the male fetus failed to respond to, one or more androgens normally produced during pregnancy. Accordingly, micropenis can often be treated with injections of various hormones, such as human chorionic gonadotropin or testosterone.

Treatment of micropenis when hormone injections fail to stimulate penile growth has been a controversial issue. In the past, doctors frequently recommended and performed sexual reassignment surgery, and the child would then be raised as a girl. However, in many cases the patients of such surgeries have rejected their female gender identity after puberty, and surveys have indicated that even those who have retained their female identity are not significantly happier with their female-ness than those who have remained outwardly male are with their male-ness. As a result, sexual reassignment is rarely performed today in response to persistent micropenis.