SETD2, an epigenetic tumor suppressor, is frequently mutated in MLL-rearranged (MLLr) leukemia and relapsed acute leukemia (AL). To clarify the impact of SETD2 mutations on chemotherapy sensitivity in MLLr leukemia, two loss-of-function (LOF) Setd2-mutant alleles (Setd2F2478L/WT or Setd2Ex6-KO/WT) were generated and introduced, respectively, to the Mll-Af9 knock-in leukemia mouse model. Both alleles cooperated with Mll-Af9 to accelerate leukemia development that resulted in resistance to standard Cytarabine-based chemotherapy. Mechanistically, Setd2-mutant leukemic cells showed downregulated signaling related to cell cycle progression, S, and G2/M checkpoint regulation. Thus, after Cytarabine treatment, Setd2-mutant leukemic cells exit from the S phase and progress to the G2/M phase. Importantly, S and G2/M cell cycle checkpoint inhibition could resensitize the Mll-Af9/Setd2 double-mutant cells to standard chemotherapy by causing DNA replication collapse, mitotic catastrophe, and increased cell death. These findings demonstrate that LOF SETD2 mutations confer chemoresistance on AL to DNA-damaging treatment by S and G2/M checkpoint defects. The combination of S and G2/M checkpoint inhibition with chemotherapy can be explored as a promising therapeutic strategy by exploiting their unique vulnerability and resensitizing chemoresistant AL with SETD2 or SETD2-like epigenetic mutations.