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Single crystals of a new ternary lanthanum magnesium antimonide LaMg0.83Sb2were successfully synthesized using the self-flux method. The crystal structure was determined by power X-ray diffraction in combination with three-dimensional electron diffraction. The crystal crystallizes in an orthorhombic-layered structure [space group C2v1‐Pmm2, Z= 2, a= 4.3692(1) Å, b= 4.3804(1) Å, c= 11.2680(3) Å]. The structure is built by alternating layers of nearly square-net 2[Sb]−and distorted anti-PbO-type 2[Mg0.83Sb]1.3–, separated by the La3+. The 2[Mg0.83Sb]1.3–layer is composed of edge-sharing MgSb4tetrahedra, which is slightly distorted from a standard anti-PbO-type tetrahedron. Magnetotransport measurements reveal a metallic resistivity with linear magnetoresistance, which can be attributed to the presence of the Dirac Fermion. First-principles calculations indicate the presence of the Dirac dispersions below the Fermi level. Our results suggest that the Dirac dispersion is a universal feature for the Sb square net, which can serve as a building block for the design of new Dirac semimetals.