AbstractThe discovery of new nonlinear optical (NLO) crystals with excellent properties is in urgently demand because of their ability to generate coherent light. Herein, we report an unique NLO lead bromide formate, KCs2[Pb2Br5 (HCOO)2], which has been synthesized by a mix‐solvothermal method. KCs2[Pb2Br5(HCOO)2] exhibits strong phase‐matching second‐harmonic generation (SHG) response (6.5×KDP), large birefringence (0.16@ 1064 nm), and a wide transparent window in most visible light and mid‐IR region. Interestingly, KCs2[Pb2Br5(HCOO)2] features a polar 3D lead‐bromide framework in which adjacent Pb−Br layers containing coplanar Pb6Br6 rings are not only parallel to each other, but also orient in the same direction. These oriented arrangements are responsible for the strong SHG response and large birefringence that are elucidated by both local dipole moment and theoretical calculations. This research provides a new strategy to explore subsequent NLO crystals.