The aberrant interaction between p53 and Mdm2/MdmX is an attractive target for cancer drug discovery because the overexpression of Mdm2 and/or MdmX ultimately impairs the function of p53 in approximately half of all human cancers. Recent studies have shown that inhibition of both Mdm2 and MdmX is more efficient than that of a single target in promoting cellular apoptosis in cancers. In this study, we demonstrate that a dual small-molecule antagonist of Mdm2/MdmX can efficiently reactivate the p53 pathway in model cancer cells overexpressing MdmX and/or Mdm2. The dual antagonist was rationally designed based on segmental mutational analysis of the N-terminal domain of MdmX and the crystal structure of the N-terminal domain of Mdm2 in complex with nutlin-3a (an Mdm2-specific inhibitor). The current work establishes a small molecule therapeutic candidate that targets cancers overexpressing Mdm2 and/or MdmX.