INTRODUCTION: Pembrolizumab, a monoclonal antibody targeting programmed cell death-1 (PD-1), is approved as a therapy for unresectable or metastatic melanoma. Immunotherapy-associated pneumonitis is an uncommon event. PATIENT CONCERNS: A 73-year-old man was admitted to our hospital with a history of melanoma on the left side of the face (resected in December 2012) and metastasis to the left lung upper lobe (resected in November 2016). Recurrence of metastasis to the bilateral lungs and left pleura was detected in April 2018. A complete response was achieved following treatment with pembrolizumab, with lower limb rashes the only adverse events occurring during therapy. The patient was readmitted in March 2019 with a productive cough, shortness of breath, and mild fever, and sputum culture identified Escherichia coli. DIAGNOSIS: A diagnosis of pneumonia was made, and although cough and shortness of breath responded to ceftazidime and levofloxacin, but fever and poor appetite persisted. Computed tomography showed no improvement in the bilateral lower lobe lesions. Prednisone was initiated based on a clinical diagnosis of immunotherapy-related pneumonitis. The response to prednisone confirmed the diagnosis. INTERVENTIONS: The patient first received ceftazidime and levofloxacin, but the symptoms persisted. Prednisone was initiated based on a clinical diagnosis of immunotherapy-related pneumonitis. OUTCOME: Complete resolution of the bilateral lung lesions occurred after 45 days of prednisone therapy. CONCLUSION: This case report highlights that both pneumonitis and bacterial pneumonia can occur as complications of anti-PD-1 immunotherapy.