Over the past 25 years, chemotherapy regimens for osteosarcoma have failed to improve the 65–70% long-term survival rate. Radiation therapy is generally ineffective except for palliative care. We here investigated whether osteosarcoma can be sensitized to radiation therapy targeting specific molecules in osteosarcoma. Large-scale RNA sequencing analysis in osteosarcoma tissues and cell lines revealed that FGFR1 is the most frequently expressed receptor tyrosine kinase in osteosarcoma. Nuclear FGFR1 (nFGFR1) was observed by subcellular localization assays. The functional studies using a FGFR1IIIb antibody or small molecule FGFR1 inhibitors showed that nFGFR1, but not membrane-bound FGFR1, induces G 2 cell-cycle checkpoint adaptation, cell survival and polyploidy following irradiation in osteosarcoma cells. Further, the activation of nFGFR1 induces Histone H3 phosphorylation at Ser 10 and c- jun /c- fos expression to contribute cell survival rendering radiation resistance. Furthermore, an in vivo mouse study revealed that radiation resistance can be reversed by the inhibition of nFGFR1. Our findings provide insights into the potential role of nFGFR1 to radiation resistance. Thus, we propose nFGFR1 could be a potential therapeutic target or a biomarker to determine which patients might benefit from radiation therapy. • Nuclear-localized FGFR1 (nFGFR1) is observed in OS tissues and cell lines. • nFGFR1 regulates radiation-induced G 2 checkpoint, cell survival and polyploidy. • The activation of nFGFR1 leads to histone modification and c-Jun/c-fos expression. • Small-molecule FGFR1 inhibitor sensitizes OS to radiation in mouse models. • nFGFR1 is suggested as a biomarker for radiation therapy in OS. [ABSTRACT FROM AUTHOR]