A 76-year-old woman was referred to our department because of high fever and bladder irritative symptoms. Computed tomography revealed the presence of a heterogeneous mass with indistinct borders on the left anterior wall of the bladder. The lesion contained a linear hyperdense shadow. We initially suspected malignancy, such as urachal carcinoma or soft-tissue sarcoma. However, upon review of previous computed tomography scans, it was confirmed that the linear hyperdense shadow had migrated from the intestinal tract to the bladder. Considering the possibility of abscess formation caused by a foreign body, we decided to perform a transurethral biopsy. The results of the pathological analysis showed abscess formation. The patient was diagnosed with perivesical abscess caused by accidental ingestion of a fish bone. Following the administration of antibiotics, the lesion markedly shrank. Although it is difficult to distinguish perivesical abscess from malignant disease, invasive treatment can be avoided by appropriate diagnosis based on imaging studies.