Hemodynamic monitoring with cardiac output (CO) assessment is usually based on invasive methods that are associated with potential complications. This study compared CO measurements during exercise obtained from a noninvasive photoplethysmography (PPG)-based chest patch monitor with those obtained from invasive thermodilution-based right heart catheterization in patients with heart failure with a preserved ejection fraction (HFpEF). CO was assessed at rest, during passive leg raises, and during 20 Watt (W) of bicycle exercise. Nine patients (67% female) with a mean age of 74.1 ± 12.1 years provided a total of 27 paired measurements. Bland-Altman analysis showed a bias of -0.72 L/min (95% CI - 1.16 to - 0.29) with 95% limits of agreement of - 2.88 to 1.43 L/min and a percentage error of 33%. The CO values obtained by thermodilution and the PPG patch were strongly correlated (r = 0.774; p < 0.001) with a concordance of r = 0.88 (p = 0.002) for the rest-to-20 W change. In this single-center feasibility study, a noninvasive PPG-based chest patch was feasible during exercise right heart catheterization and tracked exercise-induced changes in cardiac output well. Wearable monitoring may offer a low-risk, complementary approach to hemodynamic assessment, but these findings require confirmation in a larger validation cohort.