Retinoblastoma gene product (pRB) plays critical roles in regulation of the cell cycle and tumor suppression. It is known that downregulation of pRB can stimulate carcinogenesis via abrogation of the pRB pathway, although the mechanism has not been elucidated. In this study, we found that Mdm2, a ubiquitin ligase for p53, promoted ubiquitin‐dependent degradation of pRB. pRB was efficiently ubiquitinated by wild‐type Mdm2 in vivo as well as in vitro, but other RB family proteins were not. Mutant Mdm2 with a substitution in the RING finger domain showed dominant‐negative stabilization of pRB. Both knockout and knockdown of Mdm2 caused accumulation of pRB. Moreover, Mdm2 inhibited pRB‐mediated flat formation of Saos‐2 cells. Downregulation of pRB expression was correlated with a high level of expression of Mdm2 in human lung cancers. These results suggest that Mdm2 regulates function of pRB via ubiquitin‐dependent degradation of pRB.