In this study, we demonstrate that the HCMV protein pUL21a interacts with the APC, resulting in proteasome-dependent degradation of APC4 and APC5. Expression of pUL21a dissociates the APC cullin-ring ligase subcomplex from its specificity arm. This regulation alters APC activity and increases levels of a subset of APC-regulated cell cycle proteins. We have identified residues proline-arginine (PR109-110) in pUL21a to be critical for its ability to bind and regulate the APC. A mutant virus in which the viral ability to regulate the APC is abrogated by both alanine substitution of proline-arginine residues in pUL21a and UL97 deletion is markedly more defective compared to the pUL97 deletion virus alone. This suggests that HCMV has evolved an invasive strategy of using both viral factors to regulate the APC to facilitate its infection. Our study has identified the HCMV protein pUL21a as a novel APC regulator and elucidated a unique mechanism to subvert APC activity.