Actinomycosis is a very rare infection in wild animals with a few reports in captive non-human primates. Herein we report a case of pulmonary actinomycosis in a free-living black-tufted marmoset in the urban area of the Federal District, Brazil. The animal presented severe dyspnea and died in the garden of a residence. At necropsy, the left-pulmonary lobes showed multiple nodules filled with purulent content. A myriad of beaded, branching, filamentous Gram-positive and modified Ziehl-Neelsen-negative bacilli arranged in aggregates or star-like colonies, surrounded by macrophages, neutrophils, and Splendori-Hoepli phenomenon were observed in histological sections of the lungs. According to the pathological findings and characteristic morphotintorial pattern of the infectious agent, pulmonary actinomycosis was diagnosed. Until now, fatal pulmonary actinomycosis had never been reported in free-living Simiiformes. Knowledge about the diseases that affect commensal free-range simians in urban areas forms the basis for actions aimed at conservation of the species.