MicroRNA (miRNA) play essential roles in biological processes ranging from cellular proliferation to apoptosis. Recently, miRNA have also been implicated in a number of diseases including cancers. However, the targets of most miRNA remain unknown. The majority of reports describing identification of miRNA targets are based on computational approaches or detection of altered mRNA levels despite the fact that most miRNA are thought to regulate their targets primarily at the level of translational inhibition in animals. miR-21 is a miRNA with oncogenic activity that is involved in various cancer-related processes such as invasion and migration. Given the importance of miR-21 in tumorigenesis, we employed a quantitative proteomic strategy to systematically identify potential targets of miR-21. By knocking down the expression of endogenous miR-21 in MCF-7 breast cancer cells, we observed an increase in the abundance of 58 proteins, implying that they could be potential targets of miR-21. Validation of 12 of these candidate targets in luciferase assays showed that 6 of them were likely direct targets of miR-21. Importantly, the mRNA of the majority of the candidate targets tested did not show a concomitant increase in abundance. Overall, our results demonstrate that miR-21 affects the expression of many of its targets through translational inhibition and highlights the utility of proteomic approaches for identifying miRNA targets.