Purpose: We aimed to elucidate the clinical characteristics, treatment outcomes, and genetic alterations in patients with radiation-induced glioma (RIG).Methods: Patients with high-grade glioma who satisfied the Cahan’s criteria for RIG in our database during 2001–2021 were included. Isocitrate dehydrogenase (IDH) 1 and 2, telomerase reverse transcriptase (TERT) promoter, B-Raf (BRAF), and histone H3.3 (H3F3A) and methylation status of the O-6-methylguanine DNA methyltransferase (MGMT) promoter were analyzed. Methods: We identified 11 patients with RIG, including 7 glioblastoma (GBM), IDH1/2-wildtype; 2 GBM, not otherwise specified (NOS); 1 anaplastic astrocytoma (AA), IDH1/2-wildtype; and 1 AA, NOS. The median latency period was 17 years (range: 9–30 years). The median progression-free survival (PFS) and median survival time (MST) were 11.3 months and 28.3 months, respectively. The median PFS in patients treated with initial reirradiation (N = 5) tended to be longer than that in patients without initial reirradiation (N = 6) (17.0 vs 8.1 months; p = 0.51), but not MST (29.6 vs 27.4 months; p = 0.28). There were no alterations in IDH1/2, TERT promoter, BRAF and H3F3A mutations. Two tumors had a hypermethylated MGMT promoter. In one case, different IDH2 mutation status between primary and secondary tumor was useful to diagnose the secondary tumor as RIG. Conclusions: RIG may occur more than 20 years after the treatment for primary disease; therefore, long-term follow-up is needed. Reirradiation could be a therapeutic option for RIG. Identifying IDH1/2 mutation status has a diagnostic impact on establishing RIG in cases of recurrent glioma.