Because of its widespread use in furniture upholstery, buildings, and transportation, the flame retardancy of flexible polyurethane foam is a matter of great concern. In an effort to impart fire resistance in an environmentally benign manner, hexagonal boron nitride (h-BN) nanosheets were dispersed by an aqueous route and deposited as a single bilayer with polyethylenimine (PEI) on flexible polyurethane foam by using layer-by-layer assembly. Butane torch exposure and cone calorimetry show that the nanocoating performs similarly to vermiculite clay (VMT), while in situ quantitative gas phase analysis reveals that carbon monoxide emissions are lower with h-BN. The PEI/h-BN system also maintains foam flexibility and protects against weathering, preventing yellowing. These combined properties make the use of h-BN nanoplatelets an interesting alternative for the protection of polyurethane foam (and perhaps other flammable substrates).