Immune checkpoint inhibitors are associated with a wide spectrum of immune-related adverse events (irAEs) that are typically transient but are sometimes severe or even fatal. No consensus exists for the treatment of severe immune-mediated pneumonitis that is refractory to corticosteroids. Here, we report an autopsy case of pembrolizumab-induced pneumonitis that was transiently improved using infliximab. A 67-year-old male with advanced lung adenocarcinoma developed pneumonitis two weeks after a single dose of first-line pembrolizumab. The pneumonitis was refractory to corticosteroids, and the patient required mechanical ventilation. Addition of a single dose of infliximab rapidly improved the respiratory status and chest CT showed resolution of ground-glass opacities in the right upper and middle lobes. However, the patient died from re-exacerbation of pneumonitis 17 days after infliximab administration. The autopsy confirmed organizing phase diffuse alveolar damage in the right lower lobe, while the right upper lobe remained almost intact consistent with the CT findings, which is suggestive of the therapeutic effect of infliximab. The half-life of infliximab is 7-12 days, and a second dose of infliximab two weeks after the first dose is sometimes required for the treatment of gastrointestinal toxicity induced by anti-CTLA4 antibodies. Although the current guidelines do not recommend repeated administration of infliximab for immune-mediated pneumonitis, the present case suggests that repeated infliximab therapy may be beneficial in the treatment of immune-mediated pneumonitis.