2D ferroelectric (FE) materials have opened new opportunities in non-volatile memories, computation and non-linear optics due to their robust polarization in the ultra-thin limit and inherent flexibility in device integration. Recently, interest has grown in the use of 2D FEs in electro-optics, demanding the exploration of their electronic and optical properties. In this work, the discovery of an unprecedented anomalous thickness-dependent change in refractive index, as large as δn ∼ 23.2%, is reported in the 2D ferrielectric CuInP₂S₆, far above the ultra-thin limit, and at room temperature. It is also shown that the anomalous behavior in CuInP₂S₆ may be generalizable to other ferroelectric materials such as LiNbO₃. Furthermore, CuInP₂S₆ exhibits a giant birefringence in the blue-ultraviolet regime, with a maximum |n_OOP − n_IP| ∼ 1.24 at t ∼ 22 nm and λ = 339.5 nm, which is, to the best of our knowledge, the largest of any known material in this wavelength regime. Changes in the optical constants of CuInP₂S₆ are related to changes in the Cu(I) FE polarization contribution, inducing changes in its ionic mobility, and opening the door to electronic control of its optical response for use in photonics and electro-optics.