Wisteria floribunda agglutinin (WFA) is a lectin that binds to the sugar chain of Mac-2 binding protein (M2BP), and WFA-positive M2BP (WFA+-M2BP) has been reported as a useful marker for assessing liver fibrosis in chronic liver disease. Tolvaptan (TLV), a selective vasopressin V2 receptor antagonist, is used for cirrhotic ascites in Japan, but good predictors of treatment efficacy remain to be established. Our aim was to investigate whether WFA+-M2BP monitoring before and after TLV administration can predict treatment efficacy in patients with cirrhotic ascites. Twenty patients (10 men), with a median age of 72 years, were enrolled. Cirrhosis was caused by hepatitis B virus (n = 3), hepatitis C virus (n = 4), alcohol (n = 8), and others (n = 5). Responders were defined as having a body weight loss of ≥ 1.5 kg/week after TLV administration. Serum WFA+-M2BP levels were measured at baseline and days 1, 3, and 7 after TLV treatment. Twelve patients (60%) were responders. Baseline WFA+-M2BP levels were correlated with serum albumin levels (r = -0.544, P = 0.013). The baseline furosemide dose was lower and platelet count was higher in responders than in non-responders (P < 0.05). The ratio of WFA+-M2BP levels on day 1 after TLV administration to baseline was lower in responders than in non-responders (P < 0.05). The decrease in the ratio discriminated responders from non-responders (AUC = 0.844, P < 0.05). In conclusion, monitoring serum WFA+-M2BP is helpful for predicting the efficacy of TLV treatment in patients with cirrhotic ascites.