This is a 3-part study. The purpose of Part 1 of the study is to evaluate the efficacy, safety, and pharmacokinetic (PK) characteristics of safusidenib in participants with recurrent/progressive IDH1-mutant World Health Organization (WHO) Grade 2 or Grade 3 glioma. The purpose of Part 2 will be to evaluate the efficacy of maintenance safusidenib treatment versus placebo in IDH1-mutant Grade 2 or Grade 3 astrocytoma with high-risk features or IDH1-mutant Grade 4 astrocytoma, following standard-of-care radiation or chemoradiation and adjuvant temozolomide. Part 2 will be randomized, double-blind, and placebo-controlled. The purpose of Part 3 will be to evaluate the efficacy of safusidenib in participants with residual or recurrent IDH1-mutant Grade 3 oligodendroglioma who have received surgery as their only treatment. Part 3 will be an open-label single-arm cohort and will enroll participants concurrently with Part 2.