The use of implantable continuous-flow left ventricular assist devices (LVADs) as a bridge to transplant is effective for patients with congestive heart failure (HF). However, some patients develop congestive symptoms due to right-sided HF even with LVAD support. Tolvaptan, a vasopressin type 2 receptor antagonist, corrects both congestion and hyponatremia in patients with advanced HF. We report herein a case involving a patient who underwent LVAD implantation and developed hyponatremia and congestive symptoms after negative-pressure wound therapy and omental transposition for postoperative mediastinitis. Hemodynamic evaluation performed after negative-pressure wound therapy revealed elevation of both right arterial pressure and pulmonary capillary wedge pressure, and suggested biventricular dysfunction despite LVAD support. Symptoms improved after starting administration of tolvaptan. Tolvaptan may be useful for correcting hyponatremia and volume overload in patients under LVAD support.