IDH2 (isocitrate dehydrogenase 2) mutations occur in approximately 15% of patients with acute myeloid leukemia (AML). The IDH2 inhibitor enasidenib was recently approved for IDH2-mutated relapsed or refractory AML. We conducted a multi-center, phase I trial of maintenance enasidenib following allogeneic hematopoietic cell transplantation (HCT) in patients with IDH2-mutated myeloid malignancies. Two dose levels, 50mg and 100mg daily were studied in a 3x3 dose-escalation design, with 10 additional patients treated at the recommended phase 2 dose (RP2D). Enasidenib was initiated between days 30 and 90 following HCT and continued for twelve 28-day cycles. Twenty-three patients were enrolled, of whom 19 initiated post-HCT maintenance. Two had myelodysplastic syndrome, and 17 had AML. All but 3 were in first complete remission. No dose limiting toxicities were observed, and the RP2D was established at 100 mg daily. Grade ≥3 toxicities attributable to enasidenib were rare, with the most common being cytopenias. Eight patients stopped maintenance therapy before completing 12 cycles, due to adverse events (n=3), pursuing treatment for graft versus host disease (GVHD) (n=2), clinician choice (n=1), relapse (n=1), and COVID-19 infection (n=1). No cases of grade ≥3 acute GVHD were seen, and the 12-month cumulative incidence of moderate/severe chronic GVHD was 42% (20-63%). Cumulative incidence of relapse was 16% (95% CI: 3.7-36%); only one subject relapsed while receiving maintenance enasidenib. Two-year progression-free and overall survival were 69% (95% CI: 39-86%) and 74% (950% CI, 44-90%), respectively. Enasidenib is safe, well-tolerated, with preliminary activity as maintenance therapy following HCT, and merits additional study. The study was registered at ClinicalTrials.gov (NCT03515512).