A ural owl (Strix uralensis) was presented with reduced activity, emaciation and respiratory distress. The bird was severely emaciated and had an enlarged vent with a soft to fluctuant consistency. The differential blood count revealed a severe relative lymphopenia and nnonocytosis as well as a moderate leucocytosis. Radiographs showed increased opacity of the entire coelomic cavity. Ultrasonography revealed a mass with an echogenicity similar to that of liver parenchyma, with multiple cystic inclusions in the abdomen. At necropsy, the soft mass, initially suspected to be a cystic neoplasm of the liver, filled almost the entire body cavity. Histopathology of the structure identified a large blood clot demarcated by a broad band of inflammatory cells and giant cell granulomas from compressed liver tissue. Further granulomas were found in the spleen. Acid fast rods in the necrotic areas of the granulomas were visualized after Ziehl-Neelsen staining. Culture and PCR of liver and spleen demonstrated the presence of M. avium subsp. silvaticum in all samples tested. In all published cases of mycobacteriosis in Strigiformes, classical tubercles were reported. In the present case, radiographs and computed tomography, as well as gross necropsy, initially indicated a cystic neoplasia. The typical changes associated with nnycobacteriosis were only seen in histopathology and were confirmed by microbiology and PCR. M. avium subsp. silvaticum is a pathogen that may occur in immunocompetent hosts, although infection has not yet been reported in humans. Nevertheless, the possibility that the agent has a zoonotic potential cannot be excluded.