To investigate the role of miR-450b-5p, a newly identified microRNA, located in the Xq26 region, in development of chemoresistance in colorectal cancer (CRC), and to explore the underlying mechanism by which miR-450b-5p regulates this process. In this study, we demonstrated that expression of miR-450b-5p was downregulated in recurrent CRC tissues. We found that expression of miR-450b-5p was significantly inhibited in response to 5-fluorouracil (5-FU) treatment in HT-29 cells and HCT-116 cells. Importantly, overexpression of miR-450b-5p in 5-FU-resistant HT-29 cells reduced cell viability, but elevated DNA fragmentation levels and caspase-3 activity were induced by treatment with 5-FU. Conversely, inhibition of miR-450b-5p enhanced resistance to 5-FU, and promoted cell viability in HCT-116 cells. Mechanistically, we found that miR-450b-5p directly targeted SOX2, an essential factor in stem cells. Expression of miR-450b-5p was negatively correlated to the expression of SOX2, the percentages of CD133(+) cells present, and sphere-forming capacity in CRC cells. Finally, depletion of SOX2 abolished the effects of suppression of miR-450b-5p on stemness and chemoresistance in HT29 cells. We have demonstrated that miR-450b-5p inhibits stemness and development of chemoresistance to 5-FU in CRC cells. These results indicate that miR-450b-5p may be a key determinant of 5-FU sensitivity, and may represent a novel therapeutic target to facilitate chemotherapy for CRC.