Background Pneumatosis intestinalis is a rare adverse event that occurs in patients with lung cancer, especially those undergoing treatment with epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKI). Osimertinib is the most recently approved EGFR-TKI, and its usage is increasing in clinical practice for lung cancer patients who have mutations in the EGFR gene. Case presentation A 74-year-old woman with clinical stage IV (T2aN2M1b) lung adenocarcinoma was determined to have EGFR gene mutations, namely a deletion in exon 19 and a point mutation (T790 M) in exon 20. Osimertinib was started as seventh-line therapy. Follow-up computed tomography on the 97th day after osimertinib administration incidentally demonstrated intra-mural air in the transverse colon, as well as intrahepatic portal vein gas. Pneumatosis intestinalis and portal vein gas improved by fasting and temporary interruption of osimertinib. Osimertinib was then restarted and continued without recurrence of pneumatosis intestinalis. Overall, following progression-free survival of 12.2 months, with an overall duration of administration of 19.4 months (581 days), osimertinib was continued during beyond-progressive disease status, until a few days before the patient died of lung cancer. Conclusions Pneumatosis intestinalis should be noted as an important adverse event that can occur with administration of osimertinib; thus far, such an event has never been reported. This was a valuable case in which osimertinib was successfully restarted after complete recovery from pneumatosis intestinalis, such that further extended administration of osimertinib was achieved.