Updated project metadata. Heart failure with preserved ejection fraction (HFpEF) accounts for ~50% of all heart failure cases. HFpEF is a heterogeneous condition with poorly understood molecular mechanisms related to different forms including acute and chronic HFpEF. Here, we utilised plasma samples collected from patients with different forms of HFpEF to identify unique molecular mechanisms and pathways. We performed unbiased, comprehensive proteomic analyses of 31 gender- and BMI-matched plasma samples from patients with acute HFpEF (n=8), chronic HFpEF (n=9) and hypertrophic cardiomyopathy (HCM; n=14). We identified leucine-rich alpha-2-glycoprotein 1 (LRG1) as an aberrant protein across all three forms of HFpEF. We reported perturbations in signalling pathways in HFpEF mainly driven by complement/coagulation/protease and extracellular matrix (ECM) proteins, highlighting the haemostatic disturbances, notably platelet degranulation caused by systematically elevated platelet cytosolic Ca2+. We provided a publicly available online application (https://hao-chen-uts-99171821.shinyapps.io/HFpEF-Proteomics/) of our findings.