Primary small cell carcinoma of the urinary bladder is an extremely rare but important entity. We experienced a case of small cell carcinoma of the urinary bladder diagnosed by urine cytology. A 59-year-old man presented with gross hematuria and dysuria, and a calcified mass was detected at the left ureterovesical junction by cystoscopy. Abdominal ultrasonography revealed focal wall thickening at the left lateral side of the urinary bladder, and urine cytology findings were of an inflammatory background and atypical small round cells with minute hyperchromatic or pyknotic nuclei, scant cytoplasm, and rare nucleoli. In addition, atypical cells were scattered in an isolated single cell pattern or in small loose clusters with prominent nuclear molding. Subsequent histological and immunohistochemical examinations confirmed a diagnosis of small cell carcinoma.