Breast metastases from non-small cell lung carcinoma are rarely reported. We report a case of a female patient with primary adenocarcinoma of the lower lobe of her right lung presenting with a massive right-sided malignant pleural effusion. The tumor harbored an epidermal growth factor receptor insertion mutation in exon 20 but was anaplastic lymphoma kinase translocation negative. She did not respond to treatment with erlotinib. First- and second-line cytotoxic chemotherapy resulted in stable disease as the best responses. She developed right breast metastasis 20 months after her initial presentation. The rarity of the condition and the likely mechanism of the breast metastasis are discussed.