Forkhead box protein M1 (FOXM1), an important regulator of tumorigenesis in various human tumors, has recently been reported to play a role in the modulation of radiosensitivity in glioma and breast cancer cells. The present study aimed to investigate the effects of FOXM1 on radiotherapy resistance in human lung cancer and to explore the related molecular mechanisms. The results revealed that FOXM1 expression was upregulated in A549 and H1299 cells after IR (Ionizing radiation). FOXM1 inhibition impeded survival fractions, impeded proliferation, and triggered apoptosis after IR. Moreover, the silencing of FOXM1 dampened cell migration, invasion, and EMT (epithelial-mesenchyman transition) in A549 and H1299 cells treated by IR. In addition, KIF20A was also highly expressed in IR-treated A549 cells and downregulated by FOXM1 inhibition. Knockdown of KIF20A inhibited the survival fraction. Reintroduction of KIF20A partly reversed the effects of FOXM1 on the proliferation, apoptosis, and metastasis of A549 cells. Taken together, these results indicated that FOXM1 might enhance radioresistance partly through the induction of KIF20A expression.