Microplastics fibers shed from washing synthetic textiles are released directly into the waters and make up 35% of primary microplastics discharged to the aquatic environment. While there are filtration devices and legislation under way to collect these fibers, there is currently no way to safely dispose of them. Herein we investigate catalytic hydrothermal carbonization (HTC) as a means of integrating this waste (0.28 million tons of microfibers per year) into the circular economy by catalytic upcycling to carbon nanomaterials. As microfiber waste has a very high surface area, filamentous microstructure, and predictable composition (due to known fashion trends), this waste can be a precursor for high value carbon nanostructures. Herein we show that the main components of microfiber waste, cotton (natural fiber) and polyester (synthetic/polymer fiber), can be converted to filamentous solid carbon nanostructures using an Fe-Ni catalyst during HTC at 200°C and 22 bar pressure, with a very low conversion levels when a catalytic reaction hasn’t been performed.