The c-ros oncogene 1 (ROS1) fusion gene is a rare genomic alteration detected in nearly 1-2% of lung adenocarcinomas. The major partner genes of ROS1 include CD74, SDC4, and EZR. Here, we report a case of MYH9-ROS1 fusion gene-positive lung adenocarcinoma, a rare ROS1 fusion gene. The patient was a woman in her 40s who was diagnosed with advanced primary lung adenocarcinoma after a thorough examination. Initial genetic testing conducted using mediastinal lymph node biopsy specimens collected by endobronchial ultrasound-guided transbronchial needle aspiration revealed no driver gene mutations, including the ROS1 fusion gene. The patient was treated with four courses of immunochemotherapy. As the disease worsened, another genetic test was conducted using FoundationOne® CDx, and the MYH9-ROS1 fusion gene was detected. Multiple lung metastases disappeared after the administration of entrectinib; the response persisted up to a year. Adverse events of rash, dysgeusia, and peripheral edema were observed, and the patient required temporary drug interruption; however, we were able to continue entrectinib following a short-term drug interruption. This is the first report on the effectiveness of entrectinib against lung adenocarcinoma with the rare MYH9-ROS1 fusion gene.