A flame retardant hybrid coating based on montmorillonite (MMT) and carbon nanotubes (CNT) was deposited on the surface of flexible polyurethane (PU) foam by a layer-by-layer assembly method. First, chitosan-wrapped carbon nanotubes (CH-CNT) were prepared via simple mixing and ultrasound of CNT (2 mg mL(-1)) and chitosan (5 mg mL(-1)) in 1% acetic acid aqueous solution. Then, coated flexible PU foams were prepared by alternatively submerging the foam into CH-CNT suspension, MMT suspension, and alginate solution. ATR-FTIR results showed that MMT/CNT based coatings have been successfully deposited on the surface of flexible PU foam. SEM images indicated the formation of the percolated network structure between MMT and CNT. Cone calorimeter results showed that all coated flexible PU foams have lower peak heat release rates, an at least 43% reduction. This dramatically reduced fire hazard was ascribed to the MMT/CNT based network structure, which showed an excellent physical barrier effect.