In this study, a new synergistically acting intumescent flameretardant and antiseptic agent labeled ZnB-1 was synthesized based on zinc oxide, boric acid, phosphoric acid, and urea in order to provide fire protection for textile materials. The optimal temperature and reaction time parameters were determined, and the reaction yield was increased up to 92%. The chemical structure of the obtained product was investigated using FTIR spectroscopy, while its thermal stability was evaluated by TGA/DTA methods. The results confirmed the formation of a phosphorus–boron– nitrogen–zinc complex structure, the presence of covalent P–O–C bonds, and the high thermal stability and washing resistance of the flame retardant. The incorporation of ZnO nanoparticles exhibited a pronounced synergistic effect, contributing to the formation of a mineral barrier and an increased char residue yield. The synthesized flame retardant is recommended as a promising fire-protective material for cotton and cotton–polyester fabrics.