In patients hospitalized for acute decompensation of heart failure (HF), the impact of angiotensin receptor-neprilysin inhibitor (ARNI) on diuresis and renal function has not been fully investigated. Patients with HF and reduced ejection fraction who were hospitalized for acute decompensation and newly initiated ARNI after hemodynamic stabilization were enrolled. Changes in urine volume (UV), body weight, estimated glomerular filtration rate (eGFR), and urine N-acetyl-beta-d-glucosaminidase (uNAG) levels before and after ARNI initiation were investigated. Changes in the diuretic response [DR, calculated as urine volume/(intravenous furosemide volume/40 mg)], N-terminal pro-brain natriuretic peptide (NT-proBNP), hematocrit, and plasma volume (PV) were also evaluated. A total of 60 patients were enrolled. ARNI was initiated at a median of 6 [5, 7] days after hospitalization. After initiation of ARNI, body weight, NT-proBNP, and PV decreased. UV and DR increased only on the day of ARNI initiation (delta UV 400 ± 957 ml and delta DR 1100 ± 3107 ml/40 mg furosemide) and then decreased to baseline levels. In the multivariable linear regression analysis, younger age, higher BMI, and higher NT-proBNP levels were significantly associated with greater UV after ARNI initiation. eGFR and uNAG did not significantly change after the initiation of ARNI [delta eGFR -1.7 ± 12.0 mL/min/1.73 m² and delta uNAG 2.0 (−5.6, 6.9) IU/L]. In patients hospitalized for HF, the initiation of ARNI was associated with a small and transient increase in UV and DR, and was not associated with worsening of renal function or tubular injury.