We report on the in situ synthesis of nanostructured MnO₂ onto natural fique fibers. The fiber surface was rendered positive by exposure to alkaline conditions, and permanganate anions (MnO₄⁻) were embedded onto the resultant alkali cellulose via coulombic interactions. An ultrasound-assisted procedure was used to reduce MnO₄⁻ and yield MnO₂ nanoparticles (NPs). UV-Vis diffuse reflectance was used to assess the influence of the precursor concentration, loading and reduction times on the synthesis of the nanostructured MnO₂. FESEM provided direct evidence that MnO₂ NPs and aggregates could be formed on the fiber's surface. The catalytic activity of the new bionanocomposite was tested for the removal of indigo carmine dye in water samples. The MnO₂–fique fiber bionanocomposite was able to remove up to 98% of the colour present in the contaminated water samples in less than 5 minutes. Mass spectrometry was used to determine the degradation route of the dye. Additionally, we found that the bionanocomposite can be reused with no effect on the dye degradation efficiency. The reported procedure provides a new route for the development of biodegradable and easy to synthesize composite materials capable of efficiently degrading pollutants found in industrial effluents.