Herein, a unique class of post-phosphorene materials, namely, phosphorene halogenides (e.g., α-P3Cl2) with superior oxidation resistance and desirable bandgap characteristics, are proposed. Our first-principles computations show that monolayer α-P3Cl2is a direct semiconductor with a wide bandgap of 2.41 eV (HSE06) or 4.02 eV (G0W0), while the bandgap exhibits only slight reduction with increasing number of layers. The monolayer α-P3Cl2also possesses highly anisotropic carrier mobility, with both ultrahigh electron mobility (56 890 cm2V–1s–1) and hole mobility (26 450 cm2V–1s–1). Meanwhile, the outstanding optical properties and favorable band alignment of 2D P3Cl2suggest its potential as a photocatalyst for visible-light water splitting. 2D α-P3X2(X = F, Br, I) also exhibit good oxidation resistance and possess wide direct bandgaps ranging from 2.16 to 2.43 eV (HSE06). These unique electronic and optical properties render 2D phosphorene halogenide as promising functional materials for broad applications in electronic and optoelectronic devices.