Background: The venous excess ultrasound score (VExUS) is used to objectify systemic venous congestion. The aim of the paper was to determine the association between VExUS grades and worsening renal function (WRF), reduced natriuretic response, diuretics resistance, and mortality in patients with acute heart failure (AHF). Methods: One hundred patients were included, and Doppler ultrasound of hepatic, portal, and renal veins was performed. Severity of congestion was graded using the VExUS score (grade 0, 1, 2, or 3). Sodium concentration in a spot urine sample was assessed in 2 h after the first loop diuretic administration and was adjusted for the prescribed dose of furosemide (31 mmol/40 mg). Diuretics resistance was defined as the need to double the starting dose of intravenous furosemide in 6 h. Results: Patients with VExUS grade 3 showed a higher incidence of WRF (OR: 11.17; 95% CI: 3.86-32.29; p < 0.001) and a decreased natriuretic response: a spot urine sodium content of <50 mmol/L (OR: 21.53; 95% CI: 5.32-87.06; p < 0.001) and an adjusted spot urine sodium content of <31 mmol/40 mg (OR: 9.05; 95% CI: 3.15-25.96; p < 0.001). The risk of diuretic resistance (OR: 15.31; 95% CI: 5.05-46.43; p < 0.001), as well as the need for inotropic and/or vasopressor support (OR: 11.82; 95% CI: 3.59-38.92; p < 0.001), was higher in patients with severe congestion. The hospital mortality rate increased in patients with VExUS grade 3 compared to in patients with other grades (OR: 26.4; 95% CI: 5.29-131.55; p < 0.001). Conclusions: Patients with AHF and VExUS grade 3 showed a higher risk of developing WRF, a decreased diuretic and natriuretic response, a need for inotropic and/or vasopressor support, and a poor prognosis during their hospital stay.