Keloids are fibroproliferative dermal tumors of unknown origin that are characterized by the overabundant accumulation of extracellular matrix (ECM) components. The mechanism of keloid formation has remained unclear because of a poor understanding of its molecular basis. In this study, the dermal ECM components of keloids were identified, and the pathological mechanism of keloid formation was characterized using large-scale, quantitative proteomic analyses of decellularized keloid biomatrix scaffolds. We identified a total of 267 dermal core ECM and ECM-associated proteins that were differentially expressed between patients with keloids and healthy controls. Skin mechanical properties, key pathways, and biological processes including protease activity, wound healing, and adhesion were disordered in keloids. The integrated network analysis of the upregulated ECM proteins revealed the involvement of estrogen signaling pathways in keloid formation. Our findings may improve the scientific basis of keloid treatment and provide new ideas for the establishment of keloid models.