Previously, we reported that, in peritoneal dialysis (PD) patients, tolvaptan preserves residual renal function and ameliorates left ventricular hypertrophy. Here, we evaluated the effect of tolvaptan in terms of nutrition and inflammatory states. Of 24 incident PD patients with diabetes, 12 were assigned to a control group that did not receive tolvaptan, and 12, to a group that, 2 weeks after initiation of PD, received tolvaptan 15 mg daily for 12 months. At baseline and at 6 and 12 months after initiation of PD, we evaluated serum C-reactive protein (CRP), albumin, urine volume, peritoneal ultrafiltration (UF), phosphate elimination, protein uptake, left ventricular mass index (LVMI), and the diameter of the inferior vena cava (IVC). Compared with the control group, the tolvaptan group experienced preserved urine volume and UF, lower LVMI and IVC diameter, and higher protein uptake. The average protein uptake was significantly correlated with urine volume, albumin, and CRP; and serum CRP was significantly correlated with albumin. Our study results suggest that tolvaptan improves not only fluid management, but also nutrition state in PD patients.