In this study, a compact nanoscale plasmonic filter which consists of two metal–insulator–metal(MIM) waveguides coupled to each other by a rectangular ring resonator is presented andinvestigated numerically. The propagating modes of surface plasmon polaritons (SPPs) arestudied in this structure. By replacing a portion of the ring core with air, while the outerdimensions of the structure are kept constant, we illustrate the possibility of the redshift ofresonant wavelengths in order to tune the resonance modes. This feature is useful forintegrated circuits in which we have limitations on the outer dimensions of the filterstructure and it is not possible to enlarge the dimension of the ring resonator to reachlonger resonant wavelengths. The corresponding results are illustrated by the2D finite-difference time-domain (FDTD) method. The proposed structure haspotential applications in plasmonic integrated circuits and can be simply fabricated.