Right ventricular (RV) impairment after cancer therapy-related cardiotoxicity and its prognostic implications in lung cancer have not been examined. The present research sought to evaluate RV structure, function, and mechanics in stage III non-small-cell lung cancer (NSCLC) before and after concurrent chemoradiotherapy (CCRT), and to explore the associations between RV impairments, radiation dose, and all-cause mortality. This prospective investigation included 128 inoperable NSCLC patients who were scheduled to receive CCRT. Echocardiographic examination and strain evaluation was performed at baseline and 6 months post-CCRT in all participants. Conventional RV dimensions revealed no significant changes post-CCRT. However, a reduction in RV free wall strain (RV-fwLS) was observed at 6 months post-CCRT (− 28.3 ± 4.6% vs. − 25.5 ± 4.8%, P < 0.001). The same was revealed for global RV longitudinal strain (RV-GLS) (− 23.4 ± 2.9% vs. − 20.2 ± 3.4%, P < 0.001). Pearson correlation showed that RV radiation mean dose was related with the percentage change in RV-fwLS (r = 0.303, P = 0.001) and RV-GLS (r = 0.284, P = 0.002). In multivariable analysis, the percentage change in RV-fwLS was an independent predictor of all-cause mortality (HR 1.296, 95% CI 1.202–1.428, P < 0.001). RV longitudinal strain is deteriorated at 6 months post-CCRT. The RV mechanics deterioration was associated with radiation dose and affected the long-term outcome of stage III NSCLC patients treated with CCRT.