The ability to tailor the charge on cellulose-rich fibers is central to converting this important bioresource into high-end materials. However, increasing the charge often compromises the nanostructural integrity, leading to partial dissolution at high substitution levels. This work presents a new synthetic strategy for enabling cellulose-rich fibers with high and tunable charge densities (1.4–6.7 mmol/g). The method relies on radical transfer grafting via to and from polymerization of acrylic acid from thiolated fibers in water, with detailed analysis of each reaction step and how the surrounding system influences radical transfer. The resulting approach unites free radical polymerization with biopolymer science in a highly direct and versatile manner. We further show that the charged fibers perform exceptionally well in water remediation, reaching uptake values comparable to or exceeding state-of-the-art metal-organic framework (MOF) materials. This strategy offers a practical foundation for creating next-generation bio-based materials with tailored functions.