This paper presents a low-profile dual-band linear-to-circular polarizer based on aperture-coupled resonators (ACRs). The dual-band frequency response is achieved by periodically arranging the back-to-back rectangular ring and patch resonators with coupling apertures etched on the common ground. Due to the diagonal-asymmetric properties of each unit cell, quadrature phase difference between two orthogonal transmission paths with equal magnitude in each band is obtained. Thus, a 45 ^o tilted linearly polarized (LP) incident wave can be converted to a circularly polarized (CP) transmitted wave in both operation bands. To verify this concept, the proposed polarizer is designed, fabricated, and measured. Experimental results show that the proposed polarizer can achieve low insertion loss (IL) in a low-profile manner in both operation bands. The total thickness of the polarizer is about 0.09 λ ₀ , where λ ₀ is the wavelength in free space at the highest operation frequency.