  We report observation of strong and anisotropic third harmonic generation (THG) in monolayer and multilayer ReS$_2$. The third-order nonlinear optical susceptibility of monolayer ReS$_2$, $\left| \chi^{(3)} \right|$ is on the order of $10^{-18} $ m$^2$/V$^2$, which is about one order of magnitude higher than reported results for hexagonal-lattice transition metal dichalcogenides such as MoS$_2$. A similar magnitude for the third-order nonlinear optical susceptibility was also obtained for a multilayer sample. The intensity of the THG field was found to be dependent on the direction of the incident light polarization for both monolayer and multilayer samples. A point group symmetry analysis shows that such anisotropy is not expected from a perfect $1T$ lattice, and must arise from the distortions in the ReS$_2$ lattice. Our results show that THG measurements can be used to characterize lattice distortions of two-dimensional materials, and that lattice distortions are important for the nonlinear optical properties of such materials. 
Strong and Anisotropic Third Harmonic Generation in Monolayer and   Multilayer ReS$_2$