The smallest human penis ever recorded was roughly 1 cm long.
The Kinsey Institute found the smallest penis to be either 1.75 inches or 1 inch erect. However, in medical literature, there have been penises recorded to not even exceed 1 cm (roughly .39 inches). This would be a condition known as a "micropenis", an unusually small penis, that can sometimes be treated with hormonal therapy.
There is also a condition that would technically make make a penis even smaller than 1 cm. With congenital hypoplasia, the body of the penis is entirely absent. Instead, the head or glans of the penis is attached to the pubic region.
Check out World Sex Records for more on the smallest penis.