Ectopic adrenocorticotropic hormone syndrome is a paraneoplastic syndrome associated with small cell carcinoma. However, ectopic adrenocorticotropic hormone syndrome is rare in association with small cell carcinoma of the bladder. We report what we believe to be the first case of ectopic adrenocorticotropic hormone syndrome associated with small cell carcinoma of the bladder that manifested as severe muscle weakness due to hypokalemia. An early diagnosis and aggressive potassium replacement is essential for retaining muscle strength. The patient died of pneumonia less than 2 months after the diagnosis. Severe immunosuppression by excess cortisol production was suspected. Control of cortisol levels before chemotherapy might be beneficial in preventing infective complications.