Horizontal slot waveguides are becoming a very promising waveguide configuration for silicon photonics nonlinear applications. In this letter, the design of grating couplers for highly efficient coupling between standard single-mode fibers and horizontal slot waveguides is reported. Parameters have been optimized to achieve maximum coupling efficiency for lambda = 1550 nm and transverse magnetic polarization. Maximum coupling efficiencies of 61% and 48% have been achieved for negative and positive detuned grating couplers, respectively, by means of simulations. A 35-nm 1-dB bandwidth spectral response has also been obtained.