Melanin is a ubiquitous chromophore of human skin but its in vivo optical properties are relatively unexplored. We present here a detailed study of the optical absorption of melanin present in melanocytic nevi of human subjects with Fitzpatrick skin type III. Using diffuse reflectance spectroscopy, we show that the melanin absorption spectrum exhibits an exponential dependence on wavelength in vivo with a decay constant that follows a normal distribution, characteristic of a random biological variable. This is the first time such direct in vivo quantitative evidence is obtained supporting the recently proposed hypothesis of chemical and structural disorder for melanin. In addition, the ability to measure the melanin optical properties in vivo opens new ways for the study of melanin in its native environment as well as for the non-invasive study and characterization of various skin disorders and diseases.