In this paper, a class of low-profile dual-band circular polarizers based on aperture-coupled patch resonators (ACPRs) are proposed to achieve versatile linear-to-circular polarization (LP-to-CP) conversions. Initially, a dual-band dual-polarized bandpass frequency selective surface (FSS), consisting of a composite array of two independent ACPRs subelements, is presented to obtain the desired transmission properties for vertically and horizontally polarized incident waves. The equivalent circuit model is then introduced to analyze the proposed FSS, exhibiting that transmission magnitude properties of vertically- and horizontally- polarized transmitted waves can be independently designed within these two passbands. Furthermore, two pairs of rectangular slots are introduced in the patches of these ACPRs subelements, and extensively investigated to demonstrate that versatile phase differences between vertically- and horizontally- polarized transmitted waves with equal magnitude can be achieved. Based on this concept, within these two passbands, a right- or left-hand circularly polarized (RHCP or LHCP) transmitted wave is then attained by designing the vertical and horizontal components of a 45° tilted LP incident wave with equal magnitudes and ±90° phase difference, thus leading to dual-band LP-to-CP polarization conversions. To validate the design concept, two prototypes of dual-band circular polarizers with orthogonally and identically LP-to-CP conversions in each operation band are designed, fabricated, and measured, respectively. Experimental results have well revealed that our proposed structure has a few attractive advantages of ultrathin profile, low axial ratio, and flexible designs in LP-to-CP conversions and frequency band ratios.