A ternary stannide of sodium and magnesium, Na2MgSn, was synthesized from the elements, and the crystal structure was determined by single-crystal X-ray diffraction. The compound crystallizes in the Li2CuAs structure type (hexagonal, P6(3)/mmc, Z = 2, a = 5.0486(11) angstrom, c = 10.095(2) angstrom), and its structure is built up of two-dimensional honeycomb layers of (2)(infinity)[(MgSn)(2-)] stacked along the c-axis, with Na atoms as "space fillers". First-principles computations at various levels of density functional theory (DFT) verify that the most stable configuration is the one in which Na and Mg atoms occupy the 4f and 2b sites, respectively, and thus DFT provides a necessary complement to X-ray structural elucidation. Our computations also predict that Na2MgSn must be a semiconductor with a small band gap. In accord with these predictions, the electrical resistivity measured for a polycrystalline sample of Na2MgSn is 9.6-10.4 mg Omega cm in the range of 90-635 K, and the Seebeck coefficient decreases from +390 mu V K-1 (at 300 K) to +150 mu V K-1 (at 430 K).