Using density functional theory (DFT) and ab initio molecular dynamics (AIMD) we predict the existence of a new 2D monolayer, namely Penta-CoAsSe with robust mechanical, thermal and dynamical stabilities. The electronic and magnetic properties of this monolayer are investigated within the generalized gradient approximation including the Hubbard interaction (U) on the localized d orbitals of Co. We show that this material is an antiferromagnetic (AFM) narrow-gap semiconductor and exhibits in-plane magnetic anisotropy energy with a sizable magnetocrystalline anisotropy (MCA) of −1.08 erg cm⁻². Furthermore, this system is found to have a substantial intrinsic piezoelectric response with an out-of-plane coefficient d₃₆ of 0.34 pm V⁻¹, surpassing other previously reported Penta-2D piezoelectric materials. By combining our DFT calculations with the Monte Carlo simulations, we find that CoAsSe has a transition temperature four times higher than that of Penta-CoS₂. The effects of biaxial strain and electron–electron correlation on the magnetic properties and electronic structure are also examined. These fascinating properties make the Penta-CoAsSe monolayer a promising candidate for a wide range of technological applications.