Introduction/Background Durvalumab demonstrated a good efficacy and safety in patients with unresectable stage III non-small cell lung cancer (NSCLC) after concurrent chemoradiotherapy (CCRT) in the PACIFIC trial. Although a history of radiation pneumonitis (RP) has been reported to increase the risk of pneumonitis associated with programmed death-1 inhibitors, the safety and efficacy of durvalumab in patients with baseline Grade 1 RP have not been assessed. Therefore, we carried out a multicenter prospective cohort study to evaluate the efficacy and safety of durvalumab in these patients. Patients and Methods This was a multicenter prospective cohort study of 35 patients with Grade 1 RP after CCRT and before durvalumab initiation. This study was a first prespecified analysis for the first 20 patients with the primary objective of assessing the short-term safety; it was assessed 3 months after durvalumab initiation. Results Twenty patients were enrolled in this study between March 1, 2019, and September 3, 2019. Three patients (15%) experienced drug-related Grade ≥3 adverse events, while three patients (15%) had Grade ≥2 pneumonitis/RP within 3 months after durvalumab initiation. Three months after durvalumab initiation, all the patients were alive and four patients (20%) experienced disease progression. Conclusion Durvalumab can be a feasible treatment option for patients with stage III NSCLC with baseline Grade 1 RP following CCRT.(Trial registration number: UMIN000036061. The registration period was between March 2019 and December 2019.).