Most malignant melanomas are easily diagnosed; however, melanoma is also one of the lesions most frequently reported to mimic other tumors. One of the most difficult patterns to recognize is characterized by prominent myxoid matrix. A case is presented of primary cutaneous melanoma with abundant myxoid matrix in a patient who underwent prolonged phototherapy. Three years before, after getting sunburns, the patient noticed changes of a congenital nevus located in the area of sunburns. It became darker, started to blanch, and grew, with occasional bleeding. Without consulting a physician, the patient applied phototherapy onto the area for 30 months. He used a Bioptron lamp with polarized, polychromatic, incoherent light, at a wavelength from 480 to 3400 nm, without ultraviolet radiation. Clinically, the lesion was unevenly pigmented, ulcerated, covered with hemorrhagic crust, and measuring 3.5 cm in greatest dimension, with a satellite nodule. Multiple metastatic subcutaneous nodules were also found on the scalp and trunk. Histologically, the primary tumor and metastases were composed of nests and pseudotubular formations of polygonal, spindle, and stellate cells embedded in abundant myxoid stroma that comprised more than 80% of the tumor mass. Focally, in the epidermis and papillary dermis, nests of atypical melanocytes and numerous melanophages were observed. Chemotherapy and immunotherapy were administered as suggested by an oncologist. The patient died from distant metastases 6 months after the diagnosis. Although some authors believe that myxoid changes do not seem to alter the behavior of melanoma, it remains an important differential diagnosis issue.