In this study, zinc fluoroborate was synthesized by wet method using fluoroboric acid and zinc oxide as reactants, and its usability as a flame retardant for cotton fabrics was investigated. The wet method is economical, green, efficient, and applicable on a large scale. The maximum yield was related to the variation of reactant ratio, temperature, reaction time, and stirring rate. The product was characterized using Fourier-transform infrared spectroscopy, BF4- ion-selective electrode, and X-ray diffraction. The maximum yield of 97% was achieved at the reactant mole ratio (nZnO/nHBF4) of 1:3, temperature of 90 °C, and reaction time of 90 min. The purity of the synthesized product was 98%. Flame retardancy and high-temperature resistance effects of zinc fluoroborate on cotton fabrics were investigated using the limiting oxygen index (LOI) and high-temperature durability tests, respectively. Although the LOI value of the untreated original fabric was 16, this value increased to >55 upon impregnating the fabric with 50% zinc fluoroborate solution.