A plasmonic heterograting device, consisting of two juxtaposed parallel gratings with different periods, is demonstrated to function as a compact bandwidth-tunable polarization filter. The essential aspect of the structure is that the grating couples into a photonic mode of the substrate. Using this device, a linearly polarized spectrum can be conveniently and selectively picked out from nonpolarized white light. The bandwidth depends on the incident angle and the overlap of the first-order diffraction spectra of the two different grating, and can be freely narrowed. The tuning characteristics of the heterograting are investigated both theoretically and experimentally. The unique physical features potentially enable the development of new polarization elements and optical devices.