A planar compact contiguous-channel substrate integrated waveguide (SIW) multiplexer (MUX) is proposed in this paper. To reduce the size of the MUX, the output channels are closely aligned to each other through the use of a compact SIW power combiner. To develop the proposed MUX with high efficiency and high accuracy, a systematic design procedure using an aggressive space-mapping (ASM) optimization algorithm is presented. The fine model established in the full-wave simulator HFSS, the coarse model established in the circuit simulator ADS, and the ASM optimization algorithm are combined in the design procedure, and circuit elements are determined through suitable parameter-extraction procedures. As demonstrations, a Ku-band SIW four-channel MUX prototype is designed and verified by experiments.