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A 20-year-old neutered male ring-tailed lemur (Lemur catta) diagnosed with an intrathoracic mass died during surgical excision. Morphological characteristics of the mass during necropsy were consistent with a mediastinal abscess in the left hemithorax. Histological examination identified a disseminated histiocytic sarcoma involving spleen, kidneys, multiple lymph nodes, diaphragm and parietal pleura, as well as a concurrent follicular lymphoma in a separate lymph node. A mixed metastatic infiltrate composed of both neoplasms was found within the capsule of the intrathoracic abscess. Immunohistochemical analysis (Iba-1, CD3, PAX-5, IRF-4, pancytokeratin AE1/AE3 and Ki-67) confirmed both neoplasms and revealed mild IRF-4 immunoreactivity for histiocytic sarcoma cells. The histomorphologic and immunohistochemical features of the histiocytic sarcoma in this Lemur catta were characteristic of this condition in other species. This report documents the first case of histiocytic sarcoma in Lemur catta, providing novel histopathological and immunohistochemical insights of this condition in prosimians.