FAT1 cadherin exhibits dual tumor suppressor and oncogenic roles across various cancers, but its function in breast cancer remains unclear due to conflicting reports of mutational loss and overexpression. In this study, we demonstrate that FAT1 mRNA and protein levels are reduced during mammary transformation, an effect linked to promoter methylation rather than mutational events. Subtype-specific analysis reveals that high FAT1 expression correlates with poor outcomes in basal-like/triple-negative breast cancer (TNBC), while elevated FAT1 expression in luminal A/estrogen receptor-positive breast cancers is associated with improved patient prognosis. Functional studies in TNBC models using knockdown and overexpression approaches confirm that FAT1 promotes both cell proliferation and motility. High-throughput sequencing and biochemical assessments establish strong links between FAT1 phenotypes and the activation of PI3K-AKT signaling. Additionally, FAT1 manipulation induces significant changes in matrisome-related genes, extracellular matrix components, and integrin switching. Together, these findings define an oncogenic role for FAT1 in TNBC, providing mechanistic insights into how its regulation influences AKT signaling, cell proliferation, and motility.