Breast metastasis from other primary carcinoma is very rare and could be difficult to identify despite immunohistochemistry analysis. Breast metastasis from lung adenocarcinoma can mimic triple-negative breast cancer. Given the prognosis and therapeutic challenges, a correct diagnosis appears essential, and molecular biomarkers could be useful. We report the case of a 52-year-old woman with a breast mass initially diagnosed as primary breast cancer and secondarily attached to breast metastasis from an EGFR-mutated lung adenocarcinoma. The same activating EGFR mutations were identified in both the primary lung carcinoma and the breast metastasis.