Diffuse plane xanthoma (DPX) presents with symmetric yellow-orange plaques primarily on the neck, upper trunk, flexural folds, and the periorbital region. Based on serum lipid and lipoprotein levels, these xanthomas are classified as normolipemic or hyperlipoproteinemic DPX. Diffuse normolipemic plane xanthoma (DNPX) is a rare condition that is not well studied yet. It is associated with reticulo-endothelial diseases, particularly multiple myeloma and monoclonal gammopathy of unknown significance (MGUS). A 62-year-old woman developed yellowish hyperpigmented papules and diffuse patches in the medial canthal area of her neck. Based on a skin biopsy and laboratory analyses, she was diagnosed with DNPX associated with multiple myeloma. This diagnosis demonstrates that dermatological lesions should be carefully assessed as they may be the first manifestation of an underlying hematological disease. We report herein a rare case of diffuse plane xanthoma associated with multiple myeloma and review the relevant literature.