A 55-year-old woman had complained of lower abdominal pain for 5 years. A computed tomographic CT scan and magnetic resonance imaging revealed a heterogeneous density mass on the left side of the bladder. CT-guided needle biopsy showed only inflammatory cell infiltration without histological diagnosis. To confirm the diagnosis, the mass was resected together with part of the bladder and peritoneum. On the cut surface, the mass was centrally pale yellow and adherent to the thickened hard wall of the bladder and peritoneum. Pathological diagnosis was chronic granulomatous inflammation with actinomycosis. This is, to our knowledge, the 12th case report of vesical actinomycosis in Japan. Although actinomycosis is very rare in urological field, abdominal actinomycosis should be considered as the differential diagnosis of any inflammatory abdominal mass.