Extraordinary optical transmission and perfect absorption are two plasmonic-based optical phenomenona that are both accompanied by high local field enhancements that can be exploited for biological and chemical sensing. Here we merge both phenomenona in a slit array metamaterial perfect absorber consisting of a dielectric layer sandwiched between two metallic layers of which the upper layer is perforated with a periodic array of slits. Period and dimensions of the slits and thickness of the dielectric layer are tuned in a way that virtually all light of a certain wavelength incident on the surface passes through the slits and is subsequently absorbed. As a consequence, light−matter interaction is greatly enhanced in the slits. Thus, already small concentrations of analytes down to a monolayer attached to the sidewalls of the slits can be detected by refractive index sensing and identified by their spectral fingerprints with a standard mid-infrared lab spectrometer.