A novel type of absorber material integrated in a standard metal waveguide is developed for the ultra-wide frequency range of 67-500 GHz. The absorber is based on graphene augmented inorganic nanofibers which are deposited inside a metallic waveguide cassette, allowing them to be utilised in standard waveguide systems. The material’s microstructures result in a low level of reflectance (< -15 dB) and good absorbance (> 20 dB) from 110-500 GHz due to the porosity of the sample and attenuation caused by graphene, making them highly suited for wideband terahertz applications.