We propose a method for designing diffractive lenses having a fixed-position focus at several prescribed wavelengths, which we refer to as spectral diffractive lenses (SDLs). The method is based on minimizing an objective function describing the deviation of the complex transmission functions of the spectral lens at the operating wavelengths from the complex transmission functions of diffractive lenses calculated separately for each of these wavelengths. As examples, SDLs operating at three, five, and seven different wavelengths are designed. The simulation results of the calculated lenses confirm high efficiency of the proposed method. For experimental verification of the design method, we fabricate using direct laser writing and experimentally investigate an SDL operating at five wavelengths. The presented experimental results confirm the efficiency of the proposed method in practical problems of designing SDLs. The obtained results may find applications in the design and fabrication of novel flat diffractive lenses with reduced chromatic effects.