The optimal diuretic treatment strategy for patients with acute heart failure and renal dysfunction remains unclear. Plasma carbohydrate antigen 125 (CA125) is a surrogate of fluid overload and a potentially valuable tool for guiding decongestion therapy. The aim of this study was to determine if a CA125-guided diuretic strategy is superior to usual care in terms of short-term renal function in patients with acute heart failure and renal dysfunction at presentation. This multicenter, open-label study randomized 160 patients with acute heart failure and renal dysfunction into 2 groups (1:1). Loop diuretics doses were established according to CA125 levels in the CA125-guided group (n = 79) and in clinical evaluation in the usual-care group (n = 81). Changes in estimated glomerular filtration rate (eGFR) at 72 and 24 hours were the co-primary endpoints, respectively. The mean age was 78 ± 8 years, the median amino-terminal pro-brain natriuretic peptide was 7765 pg/mL, and the mean eGFR was 33.7 ± 11.3 mL/min/1.73m2. Over 72 hours, the CA125-guided group received higher furosemide equivalent dose compared to usual care (P = 0.011), which translated into higher urine volume (P = 0.042). Moreover, patients in the active arm with CA125 >35 U/mL received the highest furosemide equivalent dose (P <0.001) and had higher diuresis (P = 0.013). At 72 hours, eGFR (mL/min/1.73m2) significantly improved in the CA125-guided group (37.5 vs 34.8, P = 0.036), with no significant changes at 24 hours (35.8 vs 39.5, P = 0.391). A CA125-guided diuretic strategy significantly improved eGFR and other renal function parameters at 72 hours in patients with acute heart failure and renal dysfunction.