Soybean hulls, an abundant agro-industrial by-product, were valorized as a renewable cellulose source for nanocellulose production. Cellulose isolates were obtained by alkaline treatment followed by bleaching with sodium chlorite (NaClO₂) or hydrogen peroxide (H₂O₂) and subsequently used to produce cellulose nanocrystals (CNC) and TEMPO-oxidized cellulose nanofibrils (CNF). Process efficiency and the structural and rheological properties of the resulting nanocelluloses were systematically compared. Sodium chlorite bleaching removed lignin and hemicellulose more effectively than hydrogen peroxide treatment, yielding isolates with higher cellulose content. Consequently, CNC_NaClO2 exhibited greater aspect ratio, higher crystallinity, and improved network-forming ability. Their suspensions showed pronounced viscoelastic and thixotropic behavior (G′ > G″), whereas CNC_H2O2 displayed lower moduli and nearly Newtonian flow. CNF suspensions exhibited dominant elastic behavior and gel-like consistency regardless of bleaching method, although this effect was stronger for CNF_NaClO2 due to the formation of longer, more entangled fibrils. FTIR confirmed the high purity of all nanocelluloses, while negative surface charge ensured colloidal stability. Overall, bleaching chemistry governed nanocellulose morphology and rheological performance, enabling tailored functional properties. By maintaining a constant biomass source and identical nanostructure production conditions while varying exclusively the bleaching agent, this study implements a controlled-variable design that rigorously isolates the effect of bleaching treatment. This systematic comparative framework provides methodological robustness rarely achieved in previous studies, where differences in raw materials or processing conditions often confound interpretation.