In this article, a new synthesis method for generalized multi‐band filters has been proposed. Different from the traditional frequency transformation method, which transforms a low‐pass prototype into a multi‐band filter, multiple low‐pass prototypes have been transformed into a multi‐band filter. This study introduces the idea of transforming more than two lowpass prototypes into a filter, called multi‐prototype transformation. It divides the multi‐band filter to be synthesized into different segments each of which corresponds to a low‐pass prototype. After frequency transformation, these segments are combined together to get the final transfer function. The multi‐band filter synthesized by this method can have different orders, bandwidths or return losses in different passbands. Prescribed transmission zeros can also be realized. The return loss adjustment, segment boundary estimation and equal‐ripple realization of this method are further explained in theory. To verify this method, an asymmetric quad‐band filter is synthesized as an example and a symmetric triple‐band filter is synthesized, designed, fabricated and measured. The obtained results show good agreement with the simulation.