A new quaternary compound, AuNb4P2S20, was synthesized by direct solid-state reaction from Au, Nb, P, and S at 900 °C. Its structure was determined by the single-crystal X-ray diffraction method. AuNb4P2S20 was dark-gray crystals with structures similar to those of ANb2PS10 (A = Na, Ag). The needle-shape compound crystallizes in the monoclinic space group C2/c with cell dimensions of a = 24.5618(10) Å, b = 7.5439(11) Å, c = 12.983(3) A, β = 92.83(2)°, and Z = 4. This structure consists of one-dimensional infinite chains built by [Nb2S12] and [PS4] units. Au atoms are occupied within the van der Waals gap of sulfur atoms between infinite chains to make -S⋯Au+⋯S- contacts. There is only one Au atom site and four atoms at crystallographically equivalent sites in a unit cell. This is only the half of the numbers of Na or Ag atoms in NaNb2PS10 or AgNb2PS10. The higher oxidation state, therefore, in Au or Nb should exist to make the charge neutrality. The results of XPS measurements in AuNb4P2S20 indicate that there are two mixed oxidation states on Nb not on Au. Magnetic measurements suggest that the compound is antiferromagnetic material with the transition temperature of 45 K. The interrelationships between mixed oxidation states and electronic properties are discussed.