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.