A new concept to achieve the generalized synthesis of crystalline mesoporous rare earth (RE) oxide thin films templated with an ionic amphiphilic block copolymer is developed. Mesoporous La2O3, Eu2O3, Tb2O3, and Yb2O3 thin films as representatives of the light and heavy RE groups are synthesized by using polystyrene-block-poly(acrylic acid) (PS-b-PAA) as a templating agent. The impact of the concentration of PS-b-PAA on the morphologies of the mesopores is investigated. The local and long-range lateral structures, vertical structures, and crystallinities of the mesoporous thin films are probed by scanning electron microscopy, atomic force microscopy, grazing-incidence small-angle X-ray scattering, X-ray reflectivity, and transmission electron microscopy. The mechanism of formation of the mesoporous RE oxide thin films is discussed.