Highly efficient nonlinear optical quinolinium single crystals are newly designed and synthesized. The quinolinium single crystals consist of a 2-(4-hydroxystyryl)-1-methylquinolinium cation possessing large molecular optical nonlinearity and a 2,4,6-trimethylbenzenesulfonate counter anion. The quinolinium crystals exhibit monoclinic acentric space group P2(1) symmetry, resulting in large macroscopic nonlinear optical response, which is comparable with those of highly nonlinear optical configurationally locked polyene crystals. Plate-shaped bulk quinolinium crystals with an area of up to 56 mm(2) are successfully grown by the solution growth method. Single quinolinium crystals exhibit a large transparency range (550-1600 nm) in the optical region and a relatively low absorption coefficient in the THz region.