Background <p>In recent years, only a few cases of mucinous adenocarcinoma with signet-ring cell carcinoma (MAC-SRC) have been reported, and detailed endoscopic imaging features remain poorly characterized. This is likely because these tumors are often already at an advanced stage at diagnosis, rendering endoscopic examination infeasible. We herein report a young Asian male diagnosed with sigmoid colon MAC-SRC (60% mucinous adenocarcinoma, 40% signet-ring cell carcinoma).</p> Case presentation <p>A 27-year-old male presented with recurrent vomiting for 20 days. Abdominal computed tomography showed marked thickening of the sigmoid colon wall. Colonoscopy demonstrated luminal stenosis and rigidity, with extensive erosive lesions covered by abundant, difficult-to-irrigate white mucus; the lesion measured 8 cm in length. Biopsy confirmed mucinous adenocarcinoma. The patient underwent left hemicolectomy, and postoperative pathology verified MAC-SRC. Four months after initiating chemotherapy, the patient continues treatment with no changes in lifestyle, diet, or work compared to the pre-illness state.</p> Conclusions <p>Compared with conventional adenocarcinoma, MAC-SRC in young patients shows distinct clinical features and carries a high risk of missed diagnosis. Colonoscopy with adequate biopsy is essential. This patient initially presented with upper gastrointestinal symptoms, leading to a delay in diagnosis. Timely intervention was initiated following endoscopic detection of mucosal abnormalities and confirmatory biopsy findings. The endoscopic characteristics we describe provide diagnostic evidence for MAC-SRC and have important clinical implications.</p>