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Abstract Background The purpose of the present study was to evaluate retrospectively the efficacy and safety of proton beam therapy for elderly patients (≥80 years of age) with non-small cell lung cancer. Methods Patients diagnosed with T1–4 N0 M0 non-small cell lung cancer and treated with proton beam therapy between January 2009 and 2015 were recruited from our database retrospectively. Toxicity was evaluated using The Common Terminology Criteria for Adverse Events version 4.0. Results Thirty-five patients, including 25 (71%) with clinically inoperable lung cancer, were administered proton beam therapy. The median age was 82 years (range: 80–87 years), and the median follow-up time was 34 months (range: 10–72 months). The median dose of proton beam therapy was 80.0 Gy relative biological effectiveness (RBE) (range: 60.0–80.0 Gy [RBE]), and all patients completed the treatments. All patients were followed for at least 23 months or until their death. The 3-year overall survival rate was 67.2% (90.0% in patients with operable lung cancer, and 58.2% in those with inoperable lung cancer). The 3-year local control rate was 86.5%. Two patients presented with grade 2 pneumonitis. The occurrence rate of grade 2 pneumonitis was significantly correlated with a high lung V20 (p = 0.030), and a high mean lung dose (p = 0.030), and a low ratio of lung volume spared from 0.05 Gy (RBE) dose (total lung volume minus lung volume irradiated at least 0.05 Gy [RBE]) (p = 0.030). However, there were no cases of grade 3 or higher radiation pneumonitis. Conclusions This study suggests that the proton beam therapy was feasible for elderly patients with non-small cell lung cancer and can be considered as one of the treatment choices for elderly patients with lung cancer.