Malignant melanoma is one of the most progressive tumors in humans with increasing incidence worldwide. Dynamic nuclear polarization (DNP)-MRI is a noninvasive imaging method to obtain the spatio-temporal information of free radicals. If endogenous free radicals in melanin pigment could be utilized as a bio-probe for DNP-MRI, this will be an advantage for the specific enhancement of melanoma tissues. We report that biological melanin pigment induced a in vivo DNP effect by interacting with water molecules. In addition, we demonstrated in vivo melanoma imaging based on the DNP effects of endogenous free radicals in the melanin pigment of living mice.