Abstract Purpose: Azacitidine inhibits DNA methyltransferases, including DNMT1, and is currently the standard of care for patients with higher-risk myelodysplastic syndrome (HRMDS) or low blast count acute myeloid leukemia (AML). Experimental Design: The expression of 754 miRNAs was compared in azacitidine-resistant and azacitidine-sensitive myelodysplastic syndrome cells. We investigated the role of differentially expressed miRNAs on DNMT1 expression and azacitidine resistance in vitro. We next evaluated anti-DNMT1 miRNA expression in pretreatment bone marrow samples derived from 75 patients treated with azacitidine for HRMDS or AML. Results: Seven miRNAs, including 5 that in silico targeted the DNMT1 3′ UTR, were repressed in azacitidine-resistant cells in which DNMT1 protein levels were significantly higher. Ectopic anti-DNMT1 miRNA expression decreased DNMT1 expression and increased azacitidine sensitivity, whereas specific inhibition of endogenous anti-DNMT1 miRNAs increased DNMT1 expression and triggered azacitidine resistance. In patients treated with azacitidine, decreased expression of anti-DNMT1 miRNAs was associated with poor outcome. miR-126* had the strongest prognostic impact. Patients with miR-126*low myelodysplastic syndrome had significantly lower response rates (P = 0.04) and higher relapse rates (P = 0.03), as well as shorter progression-free (PFS; P = 0.004) and overall survival (OS; P = 0.004). Multivariate analysis showed that age, miR-126* expression, and revised International Prognostic Scoring System risk independently predicted PFS and OS. In 15 patient samples collected over time, decreased miRNA expression levels were associated with secondary resistance. Conclusions: A decreased expression of anti-DNMT1 miRNAs might account for azacitidine resistance in HRMDS and AML, and measuring miRNA expression before and during treatment might help predict primary or secondary azacitidine resistance. Clin Cancer Res; 23(12); 3025–34. ©2016 AACR.