<p>Blastic plasmacytoid dendritic cell neoplasm is a rare, aggressive hematologic malignancy that commonly involves the skin and can rapidly disseminate. We report the case of a 58-year-old man presenting with a rapidly enlarging reddish nodule over the right scapular region, which grew from 9 to 20&#xa0;cm within 1&#xa0;month. Chest magnetic resonance imaging revealed marked dermal thickening with a band-like low signal on T2-weighted images (“T2-dark band”). The T2-dark dermal layer was interposed between the tumor exhibiting restricted diffusion—superficial components bulging outward and invasive subcutaneous components (the “Hamburger Sign”). ¹⁸F-FDG PET/CT demonstrated intense uptake (SUVmax, 17) in the cutaneous lesion and revealed the involvement of multiple skeletal sites. Histopathology revealed diffuse infiltration of atypical cells extending from the dermis into the subcutis. Immunohistochemistry was positive for TCF4, CD123, and TCL1, confirming the diagnosis. Radiologic–pathologic correlation suggested that the T2-dark dermal band reflects interstitial tumor infiltration that splays and preserves dermal collagen bundles, providing a basis for the T2 shortening in a collagen-rich, low-free-water layer. We propose the “Hamburger Sign” to describe characteristic imaging findings of the blastic plasmacytoid dendritic cell neoplasm, that is, the dermal T2-dark “patty” is sandwiched by tumor “buns” and the superficial and deep tumor components. Early recognition of this sign may prompt appropriate immunohistochemical work-up (plasmacytoid dendritic markers of TCF4, CD123, and TCL1) and expedite diagnosis, particularly when initial biopsy suggests undifferentiated carcinoma.</p>