In lung squamous cell carcinoma, KEAP1 mutations frequently coexist with TP53 mutations. A preclinical model showed that mutations leading to the activation of the KEAP1-NRF2 pathway contribute to clinical radioresistance. However, there have been few clinical reports on the association between the presence of KEAP1 and TP53 mutations in patients with lung squamous cell carcinoma. Here, we report the case of a 62-year-old patient with advanced lung squamous cell carcinoma with KEAP1 and TP53 mutations who experienced primary resistance to thoracic radiotherapy. She was administered pembrolizumab in combination with cytotoxic agents as the first-line treatment and the best response was a partial response. However, the mediastinal lymph node metastases regrew 11 months after the chemotherapy. Thus, she received thoracic radiation therapy for localized lesions. However, the lesions within the radiation field had apparently progressed. Although she received subsequent chemotherapy, the lesion rapidly progressed. Treatment strategies including radiotherapy based on genetic stratification, such as KEAP1 and TP53 mutation status, should be implemented for lung squamous cell carcinoma.