We propose a plasmonic nanosensor based on Fano resonance in the strong-confinement metal–dielectric–metal waveguide side-coupled with a pair of nanoresonators. Due to the coherent interference of the splitting discrete and quasi-continuum modes, the reflection spectrum possesses a sharp asymmetric Fano resonance dip, which is dependent on the cavity–cavity phase and the refractive index change of the dielectric. The physical features contribute to a highly efficient plasmonic sensor for refractive index sensing. The nanosensor yields a sensitivity of ∼900  nm/RIU and a figure of merit of ∼500, remarkable values compared with those of plasmonic sensors supported by perfect absorbers.