The electronic energy band structure and linear optical properties of the ferroelectric semiconductor SbSI in the paraelectric phase are calculated by an ab initio pseudopotential method using density functional theory in the local density approximation. The calculated electronic band structure shows that SbSI has an indirect band gap of 1.45 eV and that the smallest direct gap is at the S point of the Brillouin zone ( 1.56 eV). The total density of states has been analysed. The linear energy dependent dielectric functions and some optical constants such as the absorption coefficient, extinction coefficient, refractive index, energy- loss function, reflectivity and optical conductivity, including self-energy effects, are calculated. The effective number of valence electrons and the effective optical dielectric constant are also calculated.