Concrete durability is strongly affected by microcracking, which facilitates the ingress of water and chloride ions and accelerates deterioration. Cellulose fibers represent a sustainable alternative to synthetic reinforcement; however, their hydrophilic nature and residual lignocellulosic components may limit their effectiveness in cementitious matrices. This study comparatively evaluates untreated and NaOH-treated cellulose fibers at volumetric dosages of 0.5% and 1.0% with respect to crack self-sealing, water permeability, and rapid chloride permeability of concrete. Alkali treatment was used to partially remove hemicellulose and lignin, increase surface roughness, and improve fiber-matrix interaction. Crack sealing performance was evaluated through time-dependent flow rate measurements, while durability characteristics were assessed using water permeability and rapid chloride permeability (RCP) tests. Results indicated that concrete incorporating treated fibers exhibited faster crack sealing and improved resistance to water and chloride penetration compared with untreated and control mixtures. The mixture containing 1.0% NaOH-treated fibers achieved complete crack sealing within approximately 528 hours and exhibited an RCP value of 1318 Coulombs, corresponding to low chloride permeability. These findings demonstrate that NaOH surface treatment significantly enhances crack sealing efficiency and reduces water and chloride permeability in cellulose fiber-reinforced concrete, confirming the potential of treated bio-based fibers as durable and sustainable reinforcement in cementitious materials.