An increasing amount of attention is being paid to guided-mode resonance (GMR) sensors since they can effectively detect small refractive indices (RIs) changes due to their narrow spectral linewidth and high efficiency. In this work, a binary, all-dielectric, asymmetric GMR (A-GMR) sensor structure with enhanced sensitivity and narrowed resonant linewidth is proposed to substantially improve the figure of merit (FOM) with respect to symmetric counterparts. To achieve the best performance, particle swarm optimization is employed to optimize the structural parameters. Furthermore, the proposed GMR sensor is compared with dielectric symmetric GMR sensors. High bulk sensitivity of 3157.6 nm/RIU, full width at half maximum down to 0.076 nm, and FOM of 41547.368 (RIU)−1 have been achieved for the A-GMR sensor, showing high-performance RI sensing.