This paper presents a detailed investigation of the electronic and optical properties of CrSe in both bulk and monolayer forms, using the ab initio method. Since half metals and spin gapless semiconductors show complete (100%) electron spin polarization around the Fermi level, they are interesting candidates for spintronic applications. The first-principles calculations used in this research focused on the electronic, optical, and elastic properties of CrSe, a binary Heusler alloy. Our analysis of the elastic constants revealed that the alloy is mechanically stable. Our electronic and magnetic properties analysis revealed that the alloy is a half-metallic (HM) ferrimagnet with a total magnetic moment (Mtot) of 4 μB/ f.u. The optimized lattice constants for the bulk and monolayer forms were 4.12 Å and 3.54 Å, respectively, and satisfy the Slater-Pauling rule (Mt=Zt- 8). Additionally, we calculated the optical spectra of the alloy in the range of energy 0-15 eV, including the actual and imaginary portions of the dielectric function, and absorption coefficient.