Background: Primary gastric melanoma (PGM) is extremely rare and often presents with non-specific gastrointestinal symptoms or incidental findings. Because there are no standardized diagnostic guidelines, confirmation by histopathology and exclusion of an extra-gastric primary site are essential.

Case presentation: A 33-year-old woman was referred for evaluation of a 4-cm mucosal elevation at the gastric antrum, initially suspected to be a subepithelial lesion. Endoscopy revealed an oval bulge with positive tenting and rolling signs and a negative cushion sign. Contrast-enhanced abdominal CT revealed a 3.7-cm, well-circumscribed, heterogeneously enhancing mass with preserved mucosa. Endoscopic ultrasonography revealed a 3.38 × 2.14-cm hypoechoic, heterogeneous lesion with irregular margins arising from the muscularis propria. Under a presumptive diagnosis of gastrointestinal stromal tumor, a laparoscopic antrectomy with Billroth-II reconstruction was performed. Histological examination revealed rhabdoid, epithelioid, and spindle cell components with scattered melanin pigments. Immunohistochemistry demonstrated positivity for HMB-45, Melan-A, PRAME, and cathepsin and negativity for S-100 and SOX-10, confirming malignant melanoma. Next-generation sequencing revealed the absence of driver mutations. Dermatological evaluation, including biopsy of a benign nevus, and whole-body 18F-FDG PET/CT excluded cutaneous or other primary origins and demonstrated no metastasis. A diagnosis of primary gastric melanoma was established. The patient recovered uneventfully and was followed up without adjuvant therapy. Conclusion: PGM can mimic gastric subepithelial lesions on endoscopy and EUS, potentially leading to misdiagnosis as gastrointestinal stromal tumors. Awareness of this rare entity, along with careful use of histopathology and immunohistochemistry, is crucial for accurate diagnosis and appropriate surgical management.