AbstractA polar tetragonal tungsten bronze, Pb1.91K3.22□0.85Li2.96Nb10O30 (□: vacancies), has been successfully synthesized by a high temperature solid‐state reaction. Single crystal and powder X‐ray diffraction indicate that the structure of Pb1.91K3.22□0.85Li2.96Nb10O30 crystallizing in the noncentrosymmetric (NCS) space group, P4bm, consists of 3D framework with highly distorted NbO6, LiO9, PbO12, and (Pb/K)O15 polyhedra. While NCS Pb1.91K3.22□0.85Li2.96Nb10O30 undergoes a reversible phase transition between polar (P4bm) and nonpolar (P4/mbm) structure at around 460 °C, the material decomposes to centrosymmetric Pb1.45K3.56Li3.54Nb10O30 (P4/mbm) once heated to 1200 °C. Powder second‐harmonic generation (SHG) measurements with 1064 nm radiation indicate that Pb1.91K3.22□0.85Li2.96Nb10O30 exhibits a giant phase‐matchable SHG intensity of ≈71.5 times that of KH2PO4, which is the strongest intensity in the visible range among all nonlinear optical materials reported to date. The observed colossal SHG should be attributable to the synergistic effect of dipole moments from the well‐aligned NbO6 octahedra, the constituting distortive channels with vacancies, and highly polarizable cations.