Malignant melanoma is a rare and high-grade cancer. It most commonly affects the skin, but it has the potential to involve all areas of the body. Primary pulmonary malignant melanoma is rare, accounting for only 0.01% of all pulmonary tumors. We present a case of primary pulmonary malignant melanoma in a 90-year-old patient. The pretreatment computed tomography (CT) showed a pulmonary mass in the right upper lobe, multiple pleural nodules, enlarged mediastinal lymph nodes, and bone metastases. Positron emission tomography-CT showed a region of fluorodeoxyglucose hyperaccumulation that was consistent with the abnormal shadows. Advanced stage lung cancer was initially suspected, but bronchoscopy revealed a malignant melanoma. The patient was diagnosed with a primary pulmonary malignant melanoma. Although the patient was older, he wanted to receive immediate treatment. Thus, he was treated with immune checkpoint inhibitors. He responded well to the medication, and neither major adverse events nor tumor size reduction was observed. We report a rare case of primary pulmonary malignant melanoma in an older adult. Immune checkpoint inhibitor therapy, as in this case, was a viable treatment option for older adults.