We report on a strong intrinsic optical anisotropy of silicon induced by dielectric nanopatterning. As a result, in-plane birefringence of anisotropically nanostructured (110) oriented Si is found to be 105 times larger than that observed in bulk silicon. The difference of the main values of the anisotropic refractive index (Δn) exceeds that of any natural birefringent crystal. Δn depends strongly on the typical size of the silicon nanowires assembling the layers and the dielectric constant of the medium surrounding these silicon nanoparticles. We show that dielectric stacks of anisotropically nanostructured Si can act as a dichroic distributed Bragg reflectors or optical microcavities. The reflection/transmission behavior of these structures is sensitive to the polarization of the incident linearly polarized light. These findings open the possibility of an application of optical devices based on birefringent silicon layers in a wide field.