A new ordered structure of Au₅Sn is determined by X-ray and electron diffraction using single crystals. The unit cell of the hexagonal superstructure contains 15 Au and 3 Sn atoms, having the lattice constants a=\sqrt3 a₀ and c=3 c₀, where a₀∼2.94 Å and c₀∼4.78 Å are those of the fundamental hcp cell. The closest interatomic distance of Sn–Sn pairs is \sqrt3 a₀∼5.1 Å, and the displacement of gold atoms from the ideal close packing is evaluated as 0.03₇ Å. The ordered phase transforms to the disordered hcp phase (ζ) at 195°C, and a revised phase diagram of the region of 10–20 at% Sn is proposed.