Canine mammary gland tumors are common and bear striking similarities to human breast cancer, making them a valuable model for comparative oncology. This study examined changes in the extracellular matrix (ECM) associated with tumor progression in spontaneous canine mammary carcinomas, aiming to identify biomarkers linked to tumor aggressiveness. Normal, non-metastatic, and metastatic mammary tissues were analyzed by histological and proteomic approaches, and human breast cancer transcriptomic data were explored for in silico validation. Histological analysis with special stains revealed a reduction in type III collagen in metastatic tumors. The proteomic profile identified 12 MEC-related proteins with differential expressions in canine samples, of which eight were upregulated (COL12A1, COL4A1, COL4A2, SERPINH1, SERPINF1, HTRA1, TNC, PCOLCE) and four (MMRN1, ABI3BP, DPT, OGN) were downregulated and were confirmed to be differentially expressed in the human transcriptome as well. The results highlight the active remodeling of the ECM during tumor progression, marked by increased expression of proteins associated with stiffness and invasiveness and decreased expression of potential suppressors. These findings strengthen the understanding of ECM dynamics in breast cancer and suggest promising molecular targets for future investigations.