More than 80% of patients with the refractory anemia with ring sideroblasts subtype of myelodysplastic syndrome (MDS) have mutations in Splicing Factor 3B, Subunit 1 (SF3B1). We generated a conditional knockin mouse model of the most common SF3B1 mutation, Sf3b1^K700E. Sf3b1^K700E mice develop macrocytic anemia due to a terminal erythroid maturation defect, erythroid dysplasia, and long-term hematopoietic stem cell (LT-HSC) expansion. Sf3b1^K700E myeloid progenitors and SF3B1-mutant MDS patient samples demonstrate aberrant 3′ splice-site selection associated with increased nonsense-mediated decay. Tet2 loss cooperates with Sf3b1^K700E to cause a more severe erythroid and LT-HSC phenotype. Furthermore, the spliceosome modulator, E7017, selectively kills SF3B1^K700E-expressing cells. Thus, SF3B1^K700E expression reflects the phenotype of the mutation in MDS and may be a therapeutic target in MDS.