Epidemiological data on death rates and incidence of Breast Cancer (BC) are still alarming, highlighting the need for new strategies of diagnosis and therapy. Our group has previously described a new antibody in Fab format, FabC4, for diagnostics, staging, and prognosis of BC. Objectives: To map the FabC4 epitopes and evaluate those for their diagnostic potential. Methods: A Phage Display-based assay against FabC4 was performed to identify and characterize peptides as new epitopes of BC. The selected peptides were also evaluated for their reactivity to patient sera through ELISA assay. We finally conducted molecular docking to assess the best conformation of FabC4-epitopes. Results: Four peptides-displaying phages differentiated sera samples from 50 patients with BC, benign disease, and from healthy women. The corresponding peptides were chemically synthesized (pA5, pA7, pC4 and pD6) and bound to FabC4. The peptide pD6 differentiated the neoplastic samples from benign and healthy sera. Molecular docking analyzes confirmed the interaction between FabC4 and the selected peptides. Conclusion: We have successfully mapped the FabC4 epitopes with diagnostic potential, opening new avenues for the understanding and treatment of BC.