We propose a planar metal-gap–dielectric waveguide for guiding modified surface plasmon polaritons (MSPPs) with a one-dimensional figure of merit of up to 3 orders of magnitude higher than that of conventional surface plasmon polaritons (SPPs). Furthermore, when a proper dielectric is filled into the gap, the MSPPs can be confined to a deep subwavelength size and simultaneously have a negligible propagation loss. More interestingly, we find that the propagation loss of the MSPPs is extremely sensitive to both the refractive index and the gap width; thus, metal-gap–dielectric waveguides are very competitive for developing highly sensitive sensors.