Tolvaptan, a diuretic with a new mechanism of action, selectively binds to the vasopressin V2 receptor and inhibits reabsorption of water. Its effect on heart failure is proven, but its benefit for patients with chronic kidney disease (CKD) has not been not confirmed. In this study, we examined the effect of tolvaptan on patients with severe CKD. We analyzed patients with stage 4 or higher CKD who had congestive heart failure that was resistant to existing diuretics. The patients were administered an initial tolvaptan dose of 7.5 mg/day. We assumed urine volume and urine osmolality to be the main effective endpoint and recorded free water clearance, serum osmolality, serum creatinine (Cr) level, and adverse events. There was no instance of clinically significant hypernatremia. The urine volume increased significantly (P < 0.0001), as did the urine osmolality (P = 0.0053). Free water clearance showed a tendency to increase, although the difference was not statistically significant. The serum creatinine level did not change significantly, and there was no clear effect on renal function. However, in patients with stage 5 CKD, the serum creatinine level decreased significantly (n = 5, P = 0.0435). There were no adverse events. We confirmed that tolvaptan has a diuretic effect in patients with both severe CKD and congestive heart failure without causing either clinically significant hypernatremia or an adverse effect on renal function. Tolvaptan is an effective diuretic for patients with CKD.