Cutaneous Rosai–Dorfman disease (CRDD), a benign histiocytosis of unknown etiology, typically presents as a solitary or clusters of lesions. Although the histopathology is fairly distinctive, the laboratory abnormalities are not; past reports note elevated erythrocyte sedimentation rate, anemia, and polyclonal hyperglobulinemia. We describe a 61-year-old African American diabetic gentleman who presented with nodules in a linear distribution on the flank. Histopathologic examination of a biopsied nodule revealed a pandermal sheet-like infiltrate of plasma cells and histiocytes, some demonstrating elastophagocytosis and emperipolesis. The lesional histiocytes were S100 and CD68 positive and CD1a negative—findings consistent with a diagnosis of CRDD. Additional laboratory work-up performed 12 weeks after the biopsy was taken revealed an elevated serum κ light chain concentration of 37.26 mg/L (reference range: 3.30–19.40 mg/L), which correlated with an M-protein spike identified as IgG κ proteins per serum protein electrophoresis. Given the difficulty in excising a large area and preexisting diabetes, a course of low-dose methotrexate was selected for therapy with a recommendation of close follow-up for the monoclonal gammopathy. To the best of our knowledge, this is the first report of CRDD associated with a linear distribution of lesions and serum protein electrophoresis–confirmed monoclonal gammopathy.