Purpose: We sought to assess clinical outcomes and toxicities of patients with recurrent lung cancer reirradiated with proton beam therapy (PBT) who were enrolled in 2 prospective registry trials. Methods and Materials: Seventy-nine consecutive patients were reirradiated with PBT at 8 institutions. Conventionally fractionated radiation therapy was used to treat the previous lung cancer in 68% of patients (median equivalent dose in 2 Gy fractions [EQD2], 60.2 Gy) and hypofractionated/stereotactic body radiation therapy in 32% (median EQD2, 83.3 Gy). Nine patients (11%) received ≥2 courses of thoracic irradiation before PBT. Eastern Cooperative Oncology Group (ECOG) performance status was 2 to 3 in 13%. Median time from prior radiation therapy to PBT was 19.9 months. PBT was delivered with conventional fractionation in 58% (median EQD2, 60 Gy), hyperfractionation in 3% (median EQD2, 62.7 Gy), and hypofractionation in 39% (median EQD2, 60.4 Gy). Twenty-four patients (30%) received chemotherapy concurrently with PBT. Results: All patients completed PBT as planned. At a median follow-up of 10.7 months after PBT, median overall survival (OS) and progression-free survival (PFS) were 15.2 months and 10.5 months, respectively. Acute and late grade 3 toxicities occurred in 6% and 1%, respectively. Three patients died after PBT from possible radiation toxicity. On multivariate analysis, ECOG performance status ≤1 was associated with OS (hazard ratio, 0.35; 95% confidence interval, 0.15-0.80; P = .014) and PFS (hazard ratio, 0.32; 95% confidence interval, 0.14-0.73; P = .007). Conclusions: This is the largest series to date of PBT reirradiation for recurrent lung cancer and indicates that reirradiation with PBT is well tolerated with acceptable toxicity and encouraging efficacy. ECOG performance status was associated with OS and PFS.