Unusual presentation of melena due to duodenal metastasis from primary lung adenocarcinoma

•A case of melena due to duodenal metastasis from primary lung adenocarcinoma.•Highlights diagnostic challenges of atypical gastrointestinal symptoms.•Emphasizes the need to consider metastatic disease in unusual presentations.•Provides insights into managing rare metastatic patterns in clinical practice. We present a rare case of duodenal metastasis from primary lung adenocarcinoma, emphasizing the diagnostic challenges and treatment considerations associated with this uncommon metastatic presentation. Through this case report, we aim to enhance awareness of this clinical entity and highlight the importance of multidisciplinary collaboration in its management.