In this study, a cellulose nanomaterial (Cellulose nanofiber, CNF) was employed as solid support and stabilizing agent for the highly active cobalt nanoparticles via in situ green and facile synthesis as an environmentally benign supported catalyst. The process of preparation involves the reduction of cobalt precursors on the surface of CNF in ambient conditions with the presence of NaBH4. The structure of the synthesized organic-inorganic hybrid Co/CNF was characterized by field emission scanning electron microscopy, energy dispersive spectroscopy and Fourier transform infrared spectroscopy. The hybrid aerogel nanocomposite Co/CNF demonstrated excellent catalytic activity for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in water with NaBH4. Even with a very low amount of catalyst and was found to be good enough to achieve 100% reduction of 4-NP with a higher reaction rate (in 8 min). The heterogeneous nanocomposite catalyst was easily isolated from the reaction mixture by simple filtration and reused 5 times without significant loss of its catalytic activity.