A novel stilbazolium benzimidazolate derivative,i.e. 2-(4-{2-[4-(octylsulfonyl)phenyl]ethenyl}pyridinio)benzimidazolate 4a, was prepared by the quaternization reaction of 4-{2-[4-(octylsulfonyl)phenyl]ethenyl}pyridine with 2-chlorobenzimidazole, followed by deprotonation with aqueous ammonia. Poly(methyl methacrylate) (PMMA) thin films containing 10 or 20 mass% of 4a were obtained by spin-coating from their THF solutions on an ordinary cover glass substrate. Second-harmonic generation (SHG) measurements of the obtained thin films were carried out by the Maker fringe method using a Q-switched Nd:YAG laser (1064 nm) as an exciting beam after corona-poling. A poled PMMA film containing 10 or 20 mass% of 4a exhibited a second-order non-linear optical (NLO) coefficient, d₃₃ , of 7.6 or 11 pm V^–1 , respectively, which was much larger than the d₃₃ value of a PMMA thin film containing 10 mass% of 2-[4-(2-phenylethenyl)pyridinio]benzimidazolate 4c (d₃₃ 1.6 pm V^–1) or 2-{4-[2-(4-octyloxyphenyl)ethenyl]pyridinio}benzimidazolate 4d (d₃₃ 0.51 pm V^–1). There were no significant differences in linear absorption properties of PMMA films containing 4a, 4c or 4d. The introduction of an octylsulfonyl group at the 4′-position of the stilbazole moiety increases the second-order NLO coefficient.