A new mercury selenide BaHgSe₂ was synthesized. This air-stable compound displays a large nonlinear optical (NLO) response and melts congruently. The structure contains chains of corner-sharing [HgSe₃]^4– anions in the form of trigonal planar units, which may serve as a new kind of basic functional group in IR NLO materials to confer large NLO susceptibilities and physicochemical stability. Such trigonal planar units may inspire a path to finding new classes of IR NLO materials of practical utility that are totally different from traditional chalcopyrite materials.