Owing to its anisotropic optical and electrical properties, rhenium diselenide (ReSe₂) has garnered considerable attention recently as a candidate material for polarization-sensitive photodetectors. However, the direct and controllable synthesis of large-sized ReSe₂ with a uniform thickness is still a great challenge. Herein, we have refined the synthesis method to obtain uniform monolayer ReSe₂ flakes with a size of up to ~ 106 μm on sapphire via an ambient-pressure chemical vapor deposition technique using Na promoter from sodium chloride. Interestingly, optical pump-probe spectroscopy revealed a fast switching from saturable absorption (SA) to absorption enhancement (AE) in subpicosecond time scale, followed by a slower decay induced by exciton recombination. Furthermore, both AE and SA signals exhibited clear angular dependence with a periodicity of 180°, which reflected the dichroism in nonlinear absorption dynamics. In addition, the photocarrier dynamics including free-carrier transport and subpicosecond relaxation due to exciton formation or surface trapping was probed using time resolved terahertz spectroscopy. We believe that our study serves as a reference for atomically controlled synthesis of large-sized ReSe₂ and provides useful insights on its optoelectronic properties for novel device applications.