LiCs2PO4, a new deep-ultraviolet (UV) transparent material, was synthesized by the flux method. The material contains unusual edge-sharing LiO4-PO4 tetrahedra. It exhibits a very short absorption edge of A = 174 nm and generates the largest powder second harmonic generation (SHG) response for deep-UV phosphates that do not contain additional anionic groups, i.e., 2.6 times that of KH2PO4 (KDP). First-principles electronic structure analyses confirm the experimental results and suggest that the strong SHG response may originate from the aligned nonbonding O-2p orbitals. The discovery and characterization of LiCs2PO4 provide a new insight into the structure property relationships of phosphate-based nonlinear optical materials with large SHG responses and short absorption edges.