New approaches for the composition of efficient robust plasmonic crystals are very desired for practical applications as well as for the observation of new exciting effects at the nanoscale. Herein, both experimentally and theoretically resonant optical effects in encapsulated magnetoplasmonic crystals composed of a 1D gold grating sandwiched between two layers of iron garnet with slightly different dielectric properties are studied. It is demonstrated that depending on the geometry of the magnetic field application, various mechanisms provide resonant enhancement of the magnetooptical (MO) response of the structure; the strongest MO effect is attained for the case of waveguide modes propagating in iron garnet dielectric layer along the longitudinal magnetic field, when the magnetization‐induced rotation of the polarization plane up to a few degrees is observed. The obtained results demonstrate high potential of magnetoplasmonic waveguides as stable and efficient plasmonic devices.