Mycosis fungoides (MF) is the most common form of cutaneous T-cell lymphoma. Persistent antigenic stimulation has been claimed to play a role in the development of this malignancy. We aimed to show the role of sulfur mustard in the pathogenesis of MF. A 45-year-old man with MF is introduced herein. He was a victim of chemical exposure in 1987 during the Iran–Iraq war. He developed skin lesions 3 years after exposure to sulfur mustard gas at the age of 21. Seven years after his exposure to sulfur mustard gas, a biopsy from the posterior distal part of his calf, which was injured and had bulla, revealed MF. Later, he developed more lesions on his extremities, trunk, and abdomen. On his previous admission, his left eyebrow was involved. A punch biopsy specimen was obtained from his eyebrow lesion, which rendered diffuse infiltration of atypical lymphocyte cells with some convoluted nuclei and scant cytoplasm admixed with lymphocytes, histiocytes, and mast cells compatible with the nodular stage of MF. At his last admission, a biopsy was obtained from the plaque lesions on his left thigh, and a TCR-γ gene rearrangement of the paraffin block of the plaque lesions revealed positive monoclonality. All the findings supported the MF diagnosis. We concluded that sulfur mustard could be a risk factor for MF development.