Multidrug resistance (MDR) is commonly correlated with the poor prognosis of chronic myelogenous leukemia. Aberrant expression of microRNAs (miRNAs) plays an important role in the MDR of various types of cancers. MicroRNA-9 (miR-9) has been confirmed to be dysregulated in many types of cancers. However, the relationship between miR-9 and MDR in chronic myelogenous leukemia (CML) remains largely unknown. In the present study, we showed that miR‑9-3p (miR-9) was downregulated in adriamycin (ADR)‑resistant K562/ADR cells and CML/MDR patients. Overexpression of miR-9 was sufficient to reverse cancer cell resistance to multiple chemotherapeutics in vitro and sensitized tumors to chemotherapy in vivo. Furthermore, we found that ABCB1 is a direct target of miR-9. These findings suggest that miR-9 plays a critical role in MDR in CML by targeting ABCB1. These results provide new evidence of miR-9 as a biomarker for clinical diagnosis and as a potential therapeutic target for CML.