Hyperpigmented mycosis fungoides (HMF) is a rare variant of mycosis fungoides (MF). Clinically it is characterized by hyperpigmented patches and plaques without evidence of regression of pre-existing lesions and poikilodermatous changes such as epidermal atrophy and telangiectasia. Histopathologic findings of HMF show marked interface change, dermal melanophages, and melanin incontinence superimposed on features of classical MF. Narrowband ultraviolet B (NBUVB) is safe and effective treatment for early MF presenting as macules or patches. The therapeutic effect of NBUVB is related to T-cell apoptosis, suppression of intercellular adhesion molecule-1, and depletion or functional modulation of Langerhans cells. A 73-year-old female patient presented with multiple hyperpigmented patches and macules on the face and neck. Histopathologic findings showed atypical lymphocytic infiltrates in epidermis and upper dermis with interface changes. In addition, melanin incontinence and melanophages were observed in the upper dermis. In immunohistochemical staining, atypical lymphocytes were positive for CD4 and CD8. On the basis of these clinical, histopathologic, and immunohistochemical findings, the diagnosis of HMF was made. The patient was treated with NBUVB phototherapy 3 times a week. A significant improvement in skin lesions was achieved after 64th NBUVB phototherapy. Herein, we report a case of HMF successfully treated with NBUVB phototherapy.