Background/Aim: MiR-221, often described both as an oncogenic microRNA and as a tumour suppressor, targets mRNAs involved in carcinogenesis. While other oncogenic microRNAs showed correlations with prostate cancer cell lines' aggressiveness, miR-221 showed an unusual overexpression in PC3. Materials and Methods: CRISPR was used to delete miR-221 from PC3 cells. Analysing the characteristics of PC3miR-221del cells, a reduced growth rate and expression of cell-cycle genes was observed. In global gene expression/ontology analysis of PC3miR-221del cells, cell-cell and cell-substrate adhesion pathways were found to be greatly affected. In addition, reduced levels of adhesion, invasion and motility for PC3miR-221del cells, a change in F-actin localisation and a reduction of EMT markers were observed. Results: The tumour suppressor gene, DIRAS3, was a predicted target of miR-221. In PC3miR-221del cells DIRAS3 was up-regulated at the gene and protein level. Ectopic expression of DIRAS3 in PC3wt cells recapitulated the cellular morphology changes seen in PC3miR-221del cells. DIRAS3 3’UTR was more stable in PC3miR-221del cells, as measured by semi-quantitative PCR and luciferase fusion reporter assays. Conclusion: MiR-221 promotes aggressiveness of PC3 cells by down-regulating DIRAS3, and promoting epithelial-to-mesenchymal transition.