Using the reductive perturbation technique, the modulational instability of ion- acoustic waves in a plasma containing superthermal electrons is studied. It is found that the presence of superthermal electrons significantly changes the instability domain. A Lorentzian (kappa) velocity distribution function is used to model superthermal electrons. It is shown that the presence of superthermal electrons reduces the critical frequency of the modulational instability of ion-acoustic waves. Besides, due to the presence of the superthermal electrons, ion-acoustic waves are unstable on a vaster region. Moreover, the modulational instability growth rate is larger for a larger population of superthermal electrons.