Background: The prevalence of pulmonary hypertension (PH) in heart failure with preserved ejection fraction (HFpEF) is increasing. We aim to study the role of big endothelin 1 (Big ET1), endothelin 1 (ET1), and neprilysin (NE) in HFpEF with PH. Method: This was a single center prospective cohort study including 90 HFpEF patients; 30 with no PH, 30 with postcapillary PH, and 30 with combined post- and precapillary PH. After enrollment, pulmonary venous and pulmonary arterial samples of Big ET1, ET1, and NE were collected during right heart catheterization. Subjects were then followed long term for adverse outcomes which included echocardiographic evidence of right ventricular dysfunction, heart failure hospitalization, and all-cause mortality. Results: Patients with HFpEF-PH were found to have increased ET1 in pulmonary veins (endothelin from the wedge; ET1W) compared to controls (2.3 ± 1.4 and 1.6 ± 0.9 pg/mL, respectively). ET1W levels were associated with both PH (OR 2.7, 95% CI 1.5–4.7, p = 0.01) and pulmonary vascular resistance (OR 1.6, 95% CI 1.04–2.3, p = 0.03). No evidence of right ventricular dysfunction was observed after 1 year of follow-up. ET1W (OR 1.8, 95% CI 1.2–2.6, p = 0.01) and ET1 gradient (ET1G; OR 1.4, 95% CI 1.04–2, p = 0.03) were predictive of 1-year hospitalization. ET1G ≥0.2 pg/mL was associated with long-term mortality (log-rank 4.8, p = 0.03). Conclusion: In HFpEF patients, ET1W and ET1G are predictive of 1-year heart failure hospitalization, while elevated ET1G levels were found to be associated with long-term mortality in HFpEF. This study highlights the role of ET1 in developing PH in HFpEF patients and also explores the potential of ET1 as a prognostic biomarker.