The development of flexible and 3D‐printable surface-enhanced Raman scattering (SERS) substrates requires hydrogel architectures that support uniform nanoparticle distribution, structural robustness, and controlled filament formation. In this study, cellulose nanofibers (CNFs) and cellulose microfibers (CMFs) were incorporated into a poly(vinyl alcohol)/sodium alginate (PVA/SA) hydrogel crosslinked through borax to elucidate how fiber geometry, interfacial chemistry, and flow behavior collectively govern printability and plasmonic performance. CNFs form an interconnected and dynamically recoverable network that enhances viscosity, elastic recovery, and structural cohesion, enabling stable extrusion during 3D printing. The shear field within the printing nozzle further induces partial alignment of CNFs, generating more continuous microdomains that influence subsequent distribution of in situ grown gold nanoparticles (AuNPs). Spectroscopic and rheological analyses show that AuNP incorporation modulates local hydrogen bonding while preserving the dynamic borate crosslinking essential for filament fidelity. The 3D-printed CNF hydrogels exhibit clear and distinguishable SERS responses, with detectable rhodamine 6G (R6G) signals down to 10−6 M. This work provides a mechanistic understanding of how fiber morphology, flow-induced alignment, and nanoparticle-matrix interactions jointly define SERS behavior in printable hydrogels, offering a scalable design framework for next‐generation soft-material sensing platforms.