A compact and wideband plasmonic directional coupler is proposed based on a hybrid metal-insulator slab waveguide (HMISW). A transfer matrix method (TMM) is adopted to analyze the multilayer structure. The hybrid waveguide is composed of a metallic layer of silver and two dielectric layers of silica (SiO2) and silicon (Si) with high index contrast. Coupling length and maximum transferred power are optimized according to dielectric layer thicknesses. It is shown that submicron coupling length is achieved with a wide bandwidth and high efficiency, being advantageous to conventional directional couplers.