Background: Concurrent chemoradiotherapy is the standard treatment for unresectable stage III non-small cell lung cancer (NSCLC). S-1 has been shown to be significant efficacious for treating advanced NSCLC. Our previous phase II study reported short-term outcomes of cisplatin (CDDP)/S-1 chemoradiotherapy. Because CDDP/S-1 chemoradiotherapy is considered to have advantages over others in overall survival (OS) and toxicity, we analyzed its long-term outcomes by following up patients included in the phase II study. Methods: Forty-eight patients (aged <75 years) with unresectable stage III NSCLC were evaluated. They were treated with CDDP (60 mg/m² on day 1) intravenously and oral S-1 (40 mg/m² twice daily on days 1–14); this regimen was repeated every 4 weeks for four cycles. A 60-Gy thoracic radiation dose was delivered in 30 fractions beginning on day 2. Results: After a median follow-up of 6.3 years (range, 5.7–7.4 years), the median OS was 2.8 years [95% confidence interval (CI); 1.04–4.63 years], and the 3- and 5-year OS rates were 49.7% (95% CI: 35.6%–63.8%) and 33.0% (95% CI: 20.0%–46.6%), respectively. Out of the several variables evaluated as predictors of OS, including gender, age, stage, histology, and performance status (PS), only PS proved to be a statistically significant predictor in both univariate and multivariate analyses. Conclusions: CDDP/S-1 concurrent thoracic radiotherapy is clinically feasible and highly efficacious. Despite our relatively small sample size, the benefits of this regimen revealed in this study warrant further research.