BACKGROUND: Heart failure with preserved ejection fraction (HFpEF) is linked to prolonged endoplasmic reticulum (ER) stress. P21-activated kinase 2 (Pak2) facilitates a protective ER stress response. This study explores the mechanism and role of Pak2 in HFpEF pathology. METHODS AND RESULTS: The HFpEF mouse model was established using a high-fat diet combined with the nitric oxide synthase inhibitor Nω-Nitro-l-arginine methyl ester (high-fat diet+Nω-Nitro-l-arginine methyl ester). The model exhibited the typical characteristics of HFpEF (cardiac hypertrophy, diastolic dysfunction with preserved systolic function, and lung edema) as determined by echocardiography and hemodynamic analysis. Terminal deoxynucleotidyl transferase dUTP nick end labeling and dihydroethidium staining results showed that cell death and reactive oxygen species generation were higher in the high-fat diet+Nω-Nitro-l-arginine methyl ester-treated group. Transmission electron microscopy revealed disruption of the ER subcellular structures in the HFpEF mouse model, while western blot analysis confirmed reduced Pak2 phosphorylation and impaired inositol-requiring enzyme 1/X-box binding protein 1 splicing ER stress response signaling. Furthermore, H9c2 cells subjected to the palmitic acid-mediated metabolic stress developed temporal changes in unfolded protein response proteins and Pak2 activity. The inositol requiring enzyme 1/X-box binding protein 1 splicing branch of unfolded protein response was impaired earlier than other branches. Overexpression of Pak2 by adenovirus in H9c2 cells sustained the activation of inositol requiring enzyme 1/X-box binding protein 1 splicing. Pak2 deficiency in the mouse heart accelerated the HFpEF progression, and this phenomenon occurred as early as 10 weeks in Pak2 cardiomyocyte-specific knockout mice. Conversely, adeno-associated virus serotype 9-mediated Pak2 overexpression mitigated HFpEF symptoms, underscoring its protective role against HFpEF progression. CONCLUSIONS: Pak2 prevents HFpEF progression, exerting cardioprotective effects against ER stress. These insights underscore the therapeutic value of Pak2 in HFpEF.