MiRNAs that are characterized by small non-coding RNAs orchestrate the expression of important genes involved in cancer cell development processes, including apoptosis, proliferation, angiogenesis, metastasis, and drug resistance at the post-transcriptional level. Dysregulation of miR-9 in various cancers has been reported. Recently, miR-9 has been considered as a key miRNA in various malignancies. However, its importance in the pathogenesis of different neoplasms is not yet well defined. Accordingly, this study was conducted in order to clarify the potential roles of miR-9 in the development of various cancers, prognosis, and treatment approaches. We have shown that a large number of miR-9 targets play fundamental roles in carcinogenesis, and that is dysregulated in various cancer cells. Our findings werefound aberrant miR-9 expression in a majority of cancers. This review article generally emphasizesthe critical roles of miR-9 in cancer cell progression. Additionally, we intended to investigate the effects of down-regulation or up-regulation of miR-9 in different types of cancers. It is hoped that a good understanding of the regulatory roles of miR-9 in various cancers ishelpful for using miR-9 in clinical settings, including prognosis, diagnosis, and miRNA-based target therapy.