Prostate cancer with refractory hyponatremia is rare. A patient was admitted with urinary retention, who developed weakness, apathy, and altered mental status during hospitalization, and was diagnosed with severe hyponatremia. After multidisciplinary consultations with departments such as endocrinology and neurology, the patient was diagnosed with syndrome of inappropriate antidiuretic hormone secretion (SIADH). The patient received serum PSA test and prostate MRI examination, and was diagnosed with prostate cancer by prostate biopsy. Laparoscopic radical prostatectomy was successfully performed. Results: The patients took tolvaptan orally before operation to maintain normal serum sodium. One month after radical prostatectomy, the symptoms of fatigue and anorexia disappeared, and serum sodium returned to normal without tolvaptan taking and sodium supplementation. No tumor recurrence or hyponatremia relapse observed during the 6-month follow-up.