In this letter, we demonstrate a refractive index sensor based on a subwavelength plasmon interferometer. Illumination of an atilt subwavelength slit-grove pair on a metal surface with monochromatic light generates high-contrast interference fringes of the transmitted light. Detection of the refractive index of the dielectric medium on the metal surface is based on examining the relative position of the interference fringes. Integration of the plasmon interferometer with a microfluidic channel provides a sensitive, high-throughput sensor with small detection volume.