The cyclin D1 proto-oncoprotein is a crucial regulator in cell-cycle progression, and aberrant overexpression of cyclin D1 is linked to tumorigenesis of many different cancer types. By screening ubiquitinated cyclin D1 as a substrate with a deubiquitinase library, we have identified USP2 as a specific deubiquitinase for cyclin D1. USP2 directly interacts with cyclin D1 and promotes its stabilization by antagonizing ubiquitin-dependent degradation. Conversely, USP2 knockdown destabilizes cyclin D1 and induces growth arrest in the human cancer lines where cell growth is dependent on cyclin D1 expression. Of note, cyclin D1 is not universally required for cell-cycle progression. Inactivation of USP2 has either very mild effects on cell growth in normal human fibroblasts or no effect in the cancer cells that do not express cyclin D1. These findings suggest that targeting USP2 is an effective approach to induce growth suppression in the cancer cells addicted to cyclin D1 expression.