Rosai-Dorfman disease or sinus histiocytosis of the lymph nodes is a rare, idiopathic, and benign disorder. Painless lymphadenopathy is the most frequent systemic presenting symptom and involves the cervical region in up to 90% of the patients. Extranodal involvement is less common and can represent the initial or the only presentation of the disease. We present the ultrasound and sonoelastographic features of a case of extranodal, pure cutaneous Rosai-Dorfman disease involving the breast, in which the breast involvement led to the final diagnosis. We discuss the ultrasound appearance of the disease (on B-mode, Doppler ultrasound, and sonoelastography) with a review of the literature and the different morphological patterns encountered in extranodal and cutaneous involvement.