Interdigitating dendritic cell sarcomas are extremely rare neoplasms with an unpredictable clinical course that usually occur in lymph nodes. We report the first adult patient with interdigitating dendritic cell sarcoma involving the bladder. A 71-year-old man presented with lower urinary tract symptoms and gross hematuria. Cystoscopy and transabdominal ultrasonography showed a large hyperechoic intravesical mass. The histologic analysis revealed a spindled cell neoplasm with an immunophenotype consistent with interdigitating dendritic cell sarcoma. Complete resection was achieved, and the patient was well without evidence of tumor 6 years after surgery. Urologists should be aware of this entity, which should be included in the differential diagnosis of bladder tumors.