In this Letter, an ultracompact terahertz (THz) mode division multiplexer based on THz spoof surface plasmon polaritons (SPPs) is proposed. Compared with traditional optical multiplexing devices, the proposed mode multiplexer can be designed with a reduced footprint by exploiting more degrees of freedom in the parameters of the unit cell, namely a rectangular metallic pillar. The ultracompact mode division multiplexer can simultaneously support the propagation of four mode channels: the TM0, TM1, TM2, and TM3 modes. Then, we numerically evaluate the performance of a cascaded plasmonic mode division circuit composed of a mode multiplexer and demultiplexer. The cross talk and excess loss of the whole circuit are lower than –15 dB and 3.7 dB, respectively, for all four mode channels at a center frequency of 0.65 THz. The footprint of the whole device is about 27 × 2.3 mm and the length of each coupling region is about 2.7 mm. For the first time, to the best of our knowledge, a mode division multiplexer based on THz spoof SPPs is reported, which will form core devices for future THz on-chip multimode communication systems.