Solar cells are expected to become one of the dominant electricity generation technologies in the coming decades. Developing high-performance absorbers made from thin materials is a promising pathway to improve efficiency and reduce cost, accelerating the widespread adoption of these photovoltaic cells. In the present work, we have systematically investigated the 2D MoSi₂N₄/Arsenene van der Waals (vdW) heterostructure, which exhibits a type-II band alignment with an indirect band gap semiconductor (1.58 eV), that can effectively separate the photogenerated electron–hole (e⁻–h⁺) pairs. Compared to the isolated MoSi₂N₄ and Arsenene monolayers, the optical absorption strength can be significantly enhanced in MoSi₂N₄/Arsenene vdW heterostructure (in the order of ∼10⁵ cm⁻¹ in the visible region). The calculated optical absorption gaps are 2.12 eV (Arsenene) and 1.76 eV (MoSi₂N₄), with excitonic binding energies of 0.05 eV for arsenene and 0.48 eV for MoSi₂N₄, indicating that both materials can effectively form excitons and separate charges. Moreover, we found a high spectroscopic limited maximum efficiency of 27.27% for the MoSi₂N₄/Arsenene vdW heterostructure, which is relatively higher compared to previously reported 2D heterostructures. Ab-initio molecular dynamics (AIMD) simulations at 300 K, 600 K, and 900 K were conducted to evaluate the thermal stability of the MoSi₂N₄/Arsenene heterostructure. Simulations in the presence of water and NO₂ at 300 K were also performed to assess its resilience to humidity and pollutants. The results suggest strong stability under harsh environmental conditions. Our findings demonstrate that the 2D MoSi₂N₄/Arsenene vdW heterostructure is an excellent candidate for both photovoltaic device applications and optoelectronic nanodevices.