Human microRNA-9 (miR-9) has been reported to be involved in the metastasis of several malignancies including brain breast cancer. However, its role in the metastasis of colorectal cancer (CRC) remains to be revealed. Here, we evaluated miR-9 expression in metastatic CRC and investigated its effects on the motility and proliferation of RKO cells. The expressions of miR-9 in 15 primary CRC specimens without distant metastasis (NM group) and 10 primary CRC specimens (M group) with distant metastasis (M group) were determined by quantitative real-time PCR. The alternations in the motility and morphology of RKO cells before and after miR-9 transfection were analyzed by migration assay and F-actin staining. The relationship between miR-9 and α-catenin was identified by Western blotting. Cell growth was examined by MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl-tetrazoliumbromide) assay. Significant difference of miR-9 expression was observed in M group compared to the NM group (P < 0.001). Ectopic expression of miR-9 enhanced the motility of RKO cells as well as changed their morphological appearance, while cell growth remained unchanged. The overexpression of miR-9 could also down-regulate α-catenin expression. These data suggest that miR-9 may potentially participate in the metastatic process of CRC though facilitating cell motility.