Acute undifferentiated leukemia (AUL) is a subtype of acute leukemias of ambiguous lineage. There is no standard treatment approach for AUL, although acute lymphoblastic leukemia–like regimens for induction therapy have been used. Additional data suggest that AUL may be better treated as acute myeloid leukemia (AML), given their similarities in genetic, cytogenetic, and gene expression patterns. Somatic mutations of IDH1 are found in 7% to 14% of patients with AML; however, the patient in this study was the first patient with IDH1-mutated AUL treated with ivosidenib. In this case, a woman aged 39 years was found to have anemia and thrombocytopenia after presenting to her primary care physician with fatigue, weight loss, and persistent infections. During further workup of the cytopenia, she was diagnosed with AUL and received 7+3 (daunorubicin, 60 mg/m2/d intravenously on days 1–3, and cytarabine, 100 mg/m2 24-hour continuous intravenous infusion on days 1–7) due to the presence of the IDH1 mutation. Bone marrow biopsy performed on day 14 of 7+3 showed persistent disease, and ivosidenib was initiated due to severe HLA alloimmunization (panel-reactive antibody, 100%) and significant bleeding complications. The patient achieved a complete morphologic and molecular remission on ivosidenib monotherapy despite critical bleeding complications during induction. Targeted therapy using ivosidenib may represent an encouraging therapeutic option in patients with AUL and IDH1 mutations. Additional evaluation of ivosidenib in this subgroup of patients with AUL is needed.