Rhenium disulphide (ReS₂) from the family of Group VII transition metal dichalcogenides (TMDs) displays unique in-plane anisotropic optical and electronic properties due to its distorted 1T’ crystal structure. Here, we present polarisation-resolved reflectance measurements performed on few-layer ReS₂ exfoliated on SiO₂/Si substrate. Using the transfer matrix method, we model and obtain the polarisation-dependent complex dielectric function of ReS₂ from these measurements_. We observe dispersion of the principal axes in the observed wavelength range. This is due to its triclinic symmetry, as well the presence of anisotropic excitonic resonances in ReS₂ conferring a wavelength dependence to all components of its dielectric tensor. This paves the way to designing polarisation sensitive photonic devices using ReS₂, and can potentially be used to explore conveniently tuneable light-matter coupling in photonic devices.