The refractive indices nx, ny, nz of nonlinear optical crystal HIO3 were determined in the wavelength range between 0.35 and 1.6μm by use of two prisms made of the crystal. The crystal was grown by means of a water solution method. We also evaluated the constants in the single-term Sellmeier dispersion relation from the measured indices. It is found that this relation is valid in the wavelength region from approximately 0.4μm to 1.2μm.