Luminescence due to the radiative recombination of localized biexcitons has been observed at low temperature (5 K) in ZnO/Zn0.74Mg0.26O multiquantum wells grown by laser-molecular-beam epilaxy on a lattice-matched ScAlMgO4 substrate (0001). The emission components due to the recombination of localized excitons and biexcitons and due to the exciton-exciton scattering were verified by examining their relative energy positions and intensity dependence on excitation power density. The excitation threshold for biexciton emission was significantly lower than that for exciton-exciton scattering. The binding energy of biexcitons in multi-quantum wells is largely enhanced by quantum confinement effect.