Introduction and importance: Malignant melanoma has a generally poor prognosis and occurs primarily on the skin but may rarely be found in internal organs such as the small intestine, colon, or rectum. Case presentation: This report presents a case of a 78-year-old male patient with stage IV gastrointestinal melanoma, which is a rare form of melanoma. The patient received first-line pembrolizumab with a complete response. Clinical discussion: Surgery plays a crucial role in local and regional control for patients with localized stages. Immune checkpoint inhibitor therapy, including nivolumab or pembrolizumab, is a well-studied and proven effective treatment option for patients with advanced skin melanoma. In this case report, the patient with gastrointestinal melanoma also had a very good response to immunotherapy. Conclusions: Understanding gastrointestinal melanoma is still limited due to the rarity of this clinical entity. Currently, there are no standard treatment guidelines for this rare group of patients. Immune checkpoint inhibitors could be the preferred first-line therapy for patients with distant metastases.