Silver nanoprisms (AgNPs) were robustly synthesized using TEMPO-oxidized cellulose nanofibrils (CNFs) as a dual capping and shape-regulating agent for the first time. Reducing AgNO3 with NaBH4 in CNF suspensions produced smaller but more uniform Ag nanospheres (AgNSs) with increasing Ag(+)/CNF ratios. CNF bound AgNSs were facilely transformed to AgNPs by etching with H2O2, supporting the capping and shape-regulating capability of CNFs. AgNPs could also be synthesized directly in a one-shot reduction reaction with NaBH4 in the presence of both CNFs and H2O2. The AgNPs transformed from CNF bound AgNSs are similar to those synthesized directly, but more stable against H2O2. Successful synthesis of AgNPs with 80-320 nm truncated edges was confirmed by light blue solution color, sharp out-of-plane quadruple resonance peak at 334 nm and prominent in-plane dipole resonance peaks at 762-900 nm. The [111] lattice plane of AgNP was clearly evident by its predominant XRD peak at 38 degrees, confirming the unique shape-regulating ability of the nearly fully surface carboxylated CNFs. The CNF surface bound AgNPs were easily fabricated into freestanding CNF/AgNPs films that showed excellent surface enhanced Raman scattering of Rhodamine 6G with analytical enhancement factor of 5 X 10(3) in contrast to none from the CNF/AgNSs film.