Methotrexate-related lymphoproliferative disorders (MTX-LPDs) are immunodeficiency diseases following methotrexate (MTX) administration, mainly occurring in rheumatoid arthritis patients. Although uncommon, MTX-LPDs have been reported in some patients with psoriasis, dermatomyositis, and cutaneous T-cell lymphoma (CTCL) who received MTX. Granulomatous mycosis fungoides (GMF) is a rare subtype of cutaneous T-cell lymphoma, where MTX is one of the treatment options in recalcitrant cases. Herein, we report a case of a 72-year-old female patient with GMF who additionally developed cutaneous Epstein-Barr virus (EBV)-positive diffuse large B-cell lymphoma (DLBCL) during MTX treatment. According to the 5th edition of the WHO classification of Haematolymphoid Tumors (WHO-HAEM), this condition is currently categorized as "lymphoma arising in immunodeficiency/dysregulation". In this article, we also reviewed published literature on cutaneous MTX-LPDs in the setting of CTCL. This entity should be considered in cases of new, atypical skin nodules and/or plaques in CTCL patients receiving long-term MTX treatment.