The refractive index of fully strained Si1−xGex layers, with compositions x=0.20, 0.25, 0.30, and 0.33, has been measured using spectroscopic ellipsometry as a function of the wavelength in the 0.9–1.7 μm range. The dependence of the refractive index on the wavelength, for these compositions, is similar to that of crystalline Si. Its value for any wavelength is lower than that of relaxed Si1−xGex of the same composition. The experimental values of the refractive index of the fully strained Si1−xGex were fitted to the expression nSiGe(x,λ)=nSi(λ)+(1.16–0.26 λ)x2. This expression is applicable for wavelengths from 0.9 to 1.7 μm and compositions x≤0.33.