Metal nanoparticles (e.g. gold nanoparticles - AuNPs) can be used in many electron-transfer reactions. With high surface-to-volume ratio and high surface energy, NPs are extremely attractive as catalysts for mild conditions reactions, i.e. at low temperatures and in aqueous systems. However, due to their small size, NPs must be immobilized in or on a support to facilitate their recovery and reuse. To increase or maintain the reaction rate, the immobilization of NPs onto the surface of monolithic porous supports is often the preferred pathway. In this work, AuNPs were synthesized under hydrothermal conditions at the surface of polyurethane foams by using an aqueous solution of chloroauric acid and a commercially available polyurethane sponge – no additional solvent or reducing agent, except water, was used. To evaluate the catalytic properties of the hybrid materials, the reduction of 4-nitrophenol to 4-aminophenol was employed. Reaction progress was monitored by UV-Vis spectroscopy. Thermogravimetric analysis and electron microscopy were employed to characterize the quantity and size of the AuNPs in the porous media. We have developed a new, facile and environmentally friendly procedure to synthesize immobilized AuNPs on porous polyurethane monoliths. The material demonstrates high catalytic properties for the reduction of 4-nitrophenol to 4-aminophenol. Finally, the method can be adapted to other metallic nanoparticles (e.g. silver and palladium).