Following the ATTRACTION-2 study, nivolumab was approved for advanced gastric cancer in Japan. However, pseudoprogression and hyperprogressive disease have been reported in patients treated with immune checkpoint inhibitors. We report a patient with gastric cancer who received nivolumab after radiotherapy only to experience rapid progression within the irradiation field after the first immunotherapy session. A 66-year-old man with dysphagia visited our hospital and was diagnosed with stage IV gastroesophageal cancer (human epidermal growth factor receptor-2 score = 0). He commenced a G-SOX regimen (S-1 80 mg/m2 on days 1-14 and oxaliplatin 100 mg/m2 on day 1, repeated every 3 weeks) in June 2017. The dysphagia worsened despite 3 cycles of G-SOX, and computed tomography (CT) revealed constriction of the gastroesophageal junction. To ameliorate the dysphagia, palliative chemoradiotherapy (S-1 and 50.4 Gy in 28 fractions) was performed starting in August 2017. The patient's dysphagia had not resolved after completing radiotherapy, and pain on swallowing worsened. Nivolumab (3 mg/m2 every 2 weeks) was administered 7 days after the completion of radiotherapy. The patient experienced malaise and worsening dysphagia before the second cycle. CT 15 days after the first nivolumab administration revealed rapid progression in the irradiation field. His general condition rapidly deteriorated, and he died 24 days after the first administration. This episode suggests that administration of nivolumab after radiotherapy may be a risk factor for hyperprogressive disease.