Rubidium melonate trihydrate Rb3[C6N7(NCN)3]·3H2O and cesium melonate trihydrate Cs3[C6N7(NCN)3]·3H2O were obtained by ion exchange reactions in aqueous solution. The structure of the cesium salt was determined by single-crystal X-ray diffraction (Pna21, a = 687.34(14), b = 2196.5(4), c = 1232.6(3) pm, V = 1860.9(7)·106 pm3, Z = 4, T = 200 K). The crystal structure of the isotypic rubidium salt was refined from X-ray powder diffraction data by the Rietveld method (Pna21, a = 674.48(2), b = 2146.4(1), c = 1207.5(1) pm, V = 1753.4(6)·106 pm3, Z = 4, T = 298 K). In the crystal structure planar melonate ions and alkali cations as well as water molecules are arranged in alternating layers. The melonate entities are interconnected by a dense hydrogen bonding network. Rb3[C6N7(NCN)3]·3H2O and Cs3[C6N7(NCN)3]·3H2O were investigated by FT-IR spectroscopy, TG and DTA measurements.