Hyponatremia is a risk factor associated with poor prognosis in patients with heart failure (HF) with reduced ejection fraction. However, whether hyponatremia has a similar role in patients with HF with preserved ejection fraction (HFpEF) has remained controversial. Therefore, the present study aimed to investigate the clinical characteristics and 24-month prognostic profile of a cohort of patients with HFpEF in China. From a registered observational cohort study on 1,027 subjects with HF, 496 patients with HFpEF were included. The association between baseline hyponatremia on admission and 24-month adverse outcomes (including all-cause mortality, re-hospitalization for HF and stroke) was analyzed using logistic regression with the Cox proportional hazards model. Of the 496 patients with HFpEF with a mean age of 72.8 years and proportion of males of 53.0%, 71 patients were diagnosed with hyponatremia. Furthermore, 29 patients (5.8%) were lost to follow-up. The hyponatremia group had lower blood pressure and serum hemoglobin, higher N-terminal pro B-type natriuretic peptide (NT-proBNP) and D-dimer, more patients with a history of atrial fibrillation and a higher proportion of spironolactone and loop diuretic use. According to a multivariate regression analysis, New York Heart Association functional classes III-IV and a serum NT-proBNP level above the median were risk factors for hyponatremia, while higher systolic blood pressure and β-blocker use were protective factors against hyponatremia. In the Kaplan-Meier analysis, hyponatremia was associated with all-causes of mortality, re-hospitalization for HF and a poor prognosis for patients suffering from strokes (log-rank P<0.05 for all 3 endpoints). On multivariate logistic regression analysis with the Cox proportional hazard model, hyponatremia was an independent predictor of three adverse outcomes [all-cause mortality: Hazard ratio (HR)=1.54, 95% CI=1.07-2.91, P=0.034; re-hospitalization for heart failure: HR=1.28, 95% CI=1.16-2.47, P=0.013; stroke: HR=1.78, 95% CI=1.04-2.89, P=0.016]. Collectively, the present results suggested that hyponatremia on admission was significantly associated with all-cause mortality, re-hospitalization and stroke within 24 months in a cohort of hospitalized patients with HFpEF in China. Thus, hyponatremia should be carefully monitored and frequently adjusted in patients with HFpEF (NCT04062500).