A method to determine the dielectric permittivity of materials is presented. Such a method exploits the use of a low-cost frequency selective structure (FSS), transversally placed in a waveguide in the proximity of the sample under test. The presence of the additional dielectric placed close to the FSS leads to a shift of bandpass and bandstop resonance frequencies. The relationship between the frequency shift and the permittivity of the dielectric under test allows the determination of the unknown permittivity. Such a procedure is particularly suitable for measuring the dielectric permittivity of thin slabs.