We propose a deterministic method to measure the optical permittivity of two-dimensional (2D) materials without a priori knowledge of the electronic transitions over the spectral window of interest. Using the thin-film approximation, we show that the ratio of reflection coefficients for s and p polarization can give a unique solution to the permittivity of 2D materials within the measured spectral window. The uniqueness and completeness of our permittivity measurement method do not require a priori knowledge of the electronic transitions of a given material. We experimentally demonstrate that the permittivity of monolayers of MoS₂, WS₂, and WSe₂ in the visible frequency range can be accurately obtained by our method. We believe that our method can provide fast and reliable measurement of the optical permittivity of newly discovered 2D materials.