Epithelial-to-mesenchymal transition (EMT) plays a crucial role in metastasis, which is the leading cause of death in breast cancer patients. We show that Cdc42 GTPase-activating protein (CdGAP) promotes tumor formation and metastasis to lungs in the HER2-positive (HER2+) murine breast cancer model. CdGAP facilitates intravasation, extravasation, and growth at metastatic sites. CdGAP depletion in HER2+ murine primary tumors mediates crosstalk with a Dlc1-RhoA pathway and is associated with a transforming growth factor-β (TGF-β)-induced EMT transcriptional signature. To further delineate the molecular mechanisms underlying the pro-migratory role of CdGAP in breast cancer cells, we searched for CdGAP interactors by performing a proteomic analysis using HEK293 cells overexpressing GFP-CdGAP. We found that CdGAP interacts with the adaptor Talin to modulate focal adhesion dynamics and integrin activation. Moreover, HER2+ breast cancer patients with high CdGAP mRNA expression combined with a high TGF-β-EMT signature are more likely to present lymph node invasion. Our results suggest CdGAP as a candidate therapeutic target for HER2+ metastatic breast cancer by inhibiting TGF-β and Integrin/Talin signaling pathways.