Updated project metadata. Objective: Obesity is associated with a pro-inflammatory and pro-thrombotic state that supports atherosclerosis progression. The goal of this study was to gain insights into the phosphorylation events related to platelet reactivity in obesity and identify platelet biomarkers and altered activation pathways in this clinical condition. Approach and Results: We performed a comparative phosphoproteomic analysis of resting platelets from obese patients and their age- and gender-matched lean controls. The phosphoproteomic data were validated by mechanistic, functional and biochemical assays. We identified 220 differentially regulated phosphopeptides, from at least 175 proteins; interestingly all were up-regulated in obesity. Most of the altered phosphoproteins are involved in Src-family kinases (SFKs)-related signaling pathways, cytoskeleton reorganization and vesicle transport, some of them validated by targeted mass spectrometry. To confirm platelet dysfunction, flow cytometry assays were performed in whole blood indicating higher surface levels of glycoprotein (GP) VI and C-type lectin-like (CLEC) -2 in platelets from obese patients correlating positively with BMI. ROC curves analysis suggested a much higher sensitivity for GPVI to discriminate between obese and lean individuals. Indeed, we also found that obese platelets displayed more adhesion to collagen-coated plates. In line with the above data, sGPVI levels - indicative of higher GPVI signaling activation - were almost double in plasma from obese patients. Conclusion: Our results provide novel information on platelet phosphorylation changes related to obesity, revealing the impact of this chronic pathology on platelet reactivity and pointing towards the main signaling pathways dysregulated.