Microwave-assisted, solid-state grafting of multiwalled carbon nanotubes is presented for the synthesis of a thermal polyurethane composite. This one-step method reduced the reaction time to the order of minutes and allowed the polyurethane to be directly attached to the nanotubes, thus forming a highly solvent-dispersible composite. The composite showed excellent optical limiting characteristics at 532 as well as 1064 nm, with a threshold of 0.10 J cm⁻² at 532 nm, and a higher value (1.0 J cm⁻²) at the latter wavelength.