A 72-year-old man presented with gross hematuria. Cystoscopy showed a non-papillary tumor at the right side of the posterior wall. Transurethral resection of the bladder tumor (TURBT) was performed. Pathologic findings demonstrated superficial transitional cell carcinoma (TCC). However, recurrent tumors were detected at the same location after 69 months' follow up. TURBT was done for the biopsy and pathologic examination showed muscle-invasive TCC. After two courses of neoadjuvant chemotherapy (MVAC), we performed radical cystectomy with Hautmann's continent reservoir. Pathologic findings revealed small cell carcinoma without any TCC features. Immunohistochemical staining using chromogranin A and synaptophysin was positive in the latest TURBT and the radical cystectomy specimens. We report a case of primary small cell carcinoma transformed from TCC of the urinary bladder.