Banana pseudostem (BPS), a widely available agricultural residue, is frequently underutilized and discarded following banana harvesting. Compositional analysis in this study revealed that BPS comprises 31.5 ± 0.1 wt% cellulose, 25.1 ± 0.5 wt% hemicellulose, 15.2 ± 0.0 wt% Klason lignin, 2.8 ± 0.3 wt% acid-soluble lignin, 11.5 ± 0.5 wt% dissolved organic matter, and 13.1 ± 0.2 wt% ash. The biomass was valorized into carboxyl cellulose nanomaterials (CCNMs) via a one-pot nitro-oxidation process, efficiently introducing carboxyl functional groups (up to 3.98 ± 0.02 mmol/g) onto the CCNM surface. Subsequently, silver nanoparticle-decorated CCNM (AgNPs/CCNM) was synthesized through the in situ reduction of AgNO₃, where CCNM served as both the reducing and stabilizing agent, thereby eliminating the need for additional reagents. Characterization confirmed the formation of AgNPs (~10 nm, 2.0 wt%) anchored on the CCNM surface. Altogether, this work highlights a sustainable approach for transforming agricultural waste into high-value, functional nanomaterial for further advanced applications.