Aims <p>Urinary sodium (UNa) is a powerful predictor of adverse events in acute heart failure (AHF), although its etiopathogenic mechanism remains poorly understood. Previous studies highlight the importance of venous congestion on renal function in AHF. Our objective was to evaluate the relationship between right ventricular (RV) function and UNa, as well as their prognostic interaction in AHF.</p> Methods <p>We prospectively analyzed 101 patients hospitalized for AHF, divided into high or low natriuresis groups based on the median natriuretic response to a furosemide stress test. We performed a comprehensive echocardiographic assessment of RV size and function. The clinical outcome was a 6-month composite of all-cause mortality, heart failure rehospitalization, or heart transplantation.</p> Results <p>The median age was 65 (56–80) years, with 63.4% of male patients. Subjects with low natriuresis showed greater RV dilation (<i>p</i> = 0.008), right atrial dilation (<i>p</i> = 0.006), inferior vena cava dilation (<i>p</i> = 0.004), higher prevalence of RV dysfunction (<i>p</i> = 0.019), and moderate-to-severe tricuspid regurgitation (<i>p</i> &lt; 0.001). RV dysfunction was associated with a higher likelihood of low natriuresis (OR 3.93 [1.72–9.02]; <i>p</i> = 0.001), poorer diuretic response during hospitalization (<i>p</i> = 0.037), and a higher risk of adverse events during follow-up (<i>p</i> = 0.002). Patients with combined RV dysfunction and low natriuresis had the worst prognosis during follow-up (HR = 38.67 [5.21–286.88]; <i>p</i> &lt; 0.001).</p> Conclusions <p>Low natriuresis is associated with a phenotype of AHF characterized by more advanced right-sided remodelling, higher prevalence of RV dysfunction, and an increased risk of diuretic resistance and adverse outcomes. RV function assessment may enhance prognostic stratification in AHF patients with impaired natriuretic response.</p> Graphical abstract <p></p>