A flame-retardant poly(acrylic acid) (FR-PAA) was prepared by copolymerization of N-(2-(5,5-dimethyl-1,3,2dioxaphosphinyl-2-ylamino)-ethylacetamide-2-propenyl acid (DPEPA) and acrylic acid and used to fabricate various FR-PAA/montmorillonite (MMT) thin films via layer-by-layer (LbL) deposition as a flame-retardant coating system for cotton fabric. Thermogravimetric analysis (TGA) indicated that treatment of FR-PAA/MMT thin films improved the thermal stability of cotton fabric. Cone calorimeter testing showed that the cotton fabrics coated with the FR-PAA/MMT thin films had less flammability with lower peak heat release rate (PHRR), lower total heat release (THR), and lower average mass loss rate (AMLR). In addition, scanning electronic microscopy (SEM) demonstrated that the surface of the fabrics coated with FR-PAA/MMT films after combustion was covered by a layer of continuous and compact char.