BACKGROUND The occurrence of metastasis from one neoplasm to another is known as tumor-to-tumor metastasis (TTM). It is a rare phenomenon in the natural history of any neoplasm, with approximately 100 cases reported in the literature to date. The lungs are the most frequent metastatic tumor donors and kidney cancer is the most common recipient. However, the opposite phenomenon (lung adenocarcinoma as a recipient of metastasis from renal carcinoma) has not been previously reported in the literature. CASE REPORT We present the case of a man with a history of multiple neoplasms. He had a diffuse large B-cell lymphoma in 2006, a left papillary renal cell carcinoma (RCC) type 2 in 2006, and an acinar adenocarcinoma of the prostate in 2011. A follow-up computed tomography scan in July 2019 showed a suspicious lung nodule on the left upper lobe and a retroperitoneal hypermetabolic mass on the positron emission tomography scan. The lung nodule and retroperitoneal mass biopsies were consistent with a primary lung adenocarcinoma with a lepidic pattern and a metastatic RCC, respectively. In January 2020, he underwent a thoracoscopic left upper lobectomy and a mediastinal lymph node dissection. Histopathological evaluation revealed a 2-cm nodule composed of a lung adenocarcinoma with an intratumoral metastasis from a papillary RCC. To date, the patient has stable renal neoplastic metastatic disease and no locoregional recurrences of the lung adenocarcinoma. CONCLUSIONS Metastasis from one primary tumor to another primary tumor is an extremely unusual event. We report one of the first cases of an RCC metastasis to a primary lung adenocarcinoma.