ReS2 exhibits strong anisotropic optical and electrical responses originating from the asymmetric lattice. Here, we show that the anisotropy of monolayer (1L) ReS2 in optical scattering and electrical transport can be practically erased by lattice engineering via lithium (Li) treatment. Scanning transmission electron microscopy revealed that significant strain is induced in the lattice of Li-treated 1L-ReS2, due to high-density electron doping and the resultant formation of continuous tiling of nanodomains with randomly rotating orientations of 60 degrees, which produced a nearly isotropic response of polarized Raman scattering and absorption of Li-treated 1L-ReS2. With Li treatment, the in-plane conductance of 1L-ReS2 increased by an order of magnitude, and its angle dependence became negligible. Our result that the asymmetric phase was converted into the isotropic phase by electron injection could significantly expand the optoelectronic applications of polymorphic two-dimensional transition metal dichalcogenides.