A new deep-ultraviolet transparent alkali metal borophosphate KNa4B2P3O13 is synthesized by high-temperature flux method, and its properties were characterized by a combination of experimental and ab initio methods. This compound crystallizes in the noncentrosymmetric Pna2(1) space group and its framework is formed of one-dimensional infinite [B2P3O13](infinity) chains consisting of apex-sharing [BO4] and [PO4] tetrahedra. KNa4B2P3O13 had a short ultraviolet edge (<190 nm) and exhibited a moderate powder second-harmonic generation signal (0.4 x KDP). The discovery of KNa4B2P3O13 enriches the structural diversity of borophosphates and would bring a more comprehensive understanding of the structure-properties relationship in deep-ultraviolet nonlinear optical crystals.