This study was conducted to impart flame retardancy to cotton fabrics while maintaining their mechanical properties and appearance under environmentally friendly and room-temperature conditions. Anionic functional groups were prepared by adding polyacrylic acid to the cotton fabric. Calcium phosphate was grafted to polyacrylic acid by inducing a reaction between calcium ions and ammonium phosphate. In this layer-by-layer (LBL) deposition, 20 layers were formed. The results of a vertical flammability test revealed that the combustion of the multilayered fabrics was delayed and that the fabric structures were maintained, even after the combustion process. Thermogravimetric analysis showed that the residues of the multilayered cotton increased to 17% at temperatures above 500 °C. X-ray photoelectron spectroscopy confirmed the production of calcium phosphate. These results indicate that calcium ions reacted with and were grafted to the carboxyl groups of polyacrylic acid, and that the grafted calcium ions reacted with ammonium phosphate to generate calcium phosphate. These findings suggest that LBL deposition can be performed with calcium phosphate to produce cotton fabrics with improved flame retardancy in an environmentally friendly manner.