BACKGROUND Pembrolizumab, a programmed cell-death protein-1 (PD-1)-targeting antibody, extends survival in cancer patients but may cause lung injury as a side effect. This immunotherapy enhances the immune system's ability to recognize and eliminate cancer cells. However, its immunomodulatory action can sometimes lead to immune-related adverse events, including lung injury. CASE REPORT A 40-year-old female patient, previously managed for malignant melanoma of the left gluteal region with surgery and immune checkpoint inhibitors, was transitioned to pembrolizumab for ongoing cancer treatment. Subsequently, she was referred to our department for further evaluation due to findings on chest imaging revealing multiple nodules in the bilateral lung fields. The patient exhibited neither cough, fever, nor breathlessness. Bronchoscopic examination yielded no abnormalities. Cytological assessments were negative, as were cultures for bacteria, fungi, and acid-fast bacilli. Bronchoalveolar lavage and endobronchial ultrasound-guided transbronchial needle aspiration biopsy of the right lower lobe bronchus B9a were conducted. Pathological analysis identified a combination of acute inflammatory and chronic fibrotic lesions, primarily histiocytic, leading to a diagnosis of pembrolizumab-induced lung injury. Steroid pulse therapy followed by tapering resulted in improvement of the pulmonary shadows. The patient is currently under observation without requiring steroid therapy. CONCLUSIONS This case underscores the importance of vigilance for potential pembrolizumab-induced lung injury in patients undergoing immunotherapy for cancer treatment. Prompt recognition and appropriate management are essential for optimizing patient outcomes. Additionally, this case highlights the challenge of diagnosing lung lesions based solely on imaging, necessitating bronchoscopy with tissue sampling as a critical diagnostic tool.