A new selenide with a diamond-like structure, Li MnSnSe , was synthesized for the first time by using a conventional high-temperature solid-state reaction method. Li MnSnSe crystallizes in the space group Pmn2 (no. 31) of the orthorhombic system. Its three-dimensional framework is constructed by corner-sharing LiSe , MnSe , and SnSe tetrahedra. The title compound has been discovered to have both type I phase-matchable behavior and to exhibit moderate powder second-harmonic generation intensity, about 0.5 times that of commercial AgGaS in the particle size of 200-250 μm at a laser radiation of 2.09 μm. In addition, Li MnSnSe exhibits congruent melting behavior, which makes the bulk single-crystal growth by the Bridgman-Stockbarger method possible. The temperature-dependent susceptibility measurement indicates an antiferromagnetic interaction with a Néel temperature (T ) of 8.6 K for this compound.