Single crystals of K3Cu2O4 were prepared by the azide/nitrate route from respective stoichiometric mixtures of KN3, KNO3 and CuO, at 923 K, whereas powder samples were synthesised by solid state reaction of K2O, KCuO2 and CuO, sealed in gold ampoules and treated at 723 K. According to the single crystal structure analysis (Cmcm, Z = 4, a = 6.1234(1), b = 8.9826(2), c = 10.8620(2) Å, R1 = 0.044, R2 = 0.107), the main structural feature are undulating CuO2 chains built up from planar, edge sharing CuO4 square units. From an analysis of the Cu–O bond lengths, the valence state of either +2 or +3 can be unambiguously assigned to each copper atom. The magnetic susceptibilities show the dominance of antiferromagnetic (AFM) interactions. At high temperatures, the magnetic behaviour can be fitted with the Curie–Weiss law (μeff = 1.84μB, Θ = –105 K). The experimental data can be very well described by a uniform Heisenberg chain with a nearest-neighbour spin intrachain interaction (Jnn) of ~ 101 K.