Background Dysregulation of miRNAs is critically implicated in tumorigenesis, and aberrant expression of miR-491-5p has been reported to play a key role in initiation and progression of various cancers. However, the biological function and underlying mechanism of miR-491-5p in colorectal cancer (CRC) remain elusive. Methods Quantitative real-time PCR (qRT-PCR) was employed to evaluate the levels of miR-491-5p and IGF2 mRNA expression in CRC tissues, cell lines and plasma. Cell counting kit-8 and colony formation assays were used to detect the effects of miR-491-5p on CRC cell growth. Luciferase reporter assays were applied to confirm the miR-491-5p target gene. In vivo experiments were conducted in nude mice. Results miR-491-5p was found to be obviously downregulated in CRC tissues and cell lines, and decreased miR-491-5p expression level was shown to be associated with differentiation, TNM stage and poor overall survival (OS). miR-491-5p overexpression suppressed CRC cell proliferation both in vitro and in vivo. Mechanically, insulin-like growth factor 2 (IGF2) was identified to be a direct target of miR-491-5p in CRC cells, and overexpression of IGF2 rescued the miR-491-5p-induced suppression of proliferation in CRC cells. Finally, we demonstrated that plasma miR-491-5p expression was decreased in CRC when compared to healthy controls and might be an effective diagnostic biomarker for CRC. Conclusion These data showed that miR-491-5p functioned as a tumor suppressor by targeting IGF2 in CRC, and miR-491-5p could serve as a potential diagnostic and prognostic biomarker for CRC.