Environmental sustainability and resource efficiency are of paramount concern in today's world. One underexplored area of sustainability lies in the valorization of agricultural wastes for the manufacturing of cellulose nanomaterials, which has relevance in several fields such as food packaging, biomedical, electronics, textiles, and several others. Considering the growing demand and variety of application, this study aims to develop environmentally friendly strategies by employing water as solvents under high-pressure fluid conditions to synthesize cellulose nanomaterials from agricultural waste. The influence of pressure (up to 150 bar), temperature (up to 200°C), and extraction time (up to 2 h) on synthesis efficiency and product quality was investigated. Results revealed that the resulting cellulose nanomaterials exhibited desirable properties with more than 50% of the particles having a size of less than 100 nm. Furthermore, a zeta potential of less than -30 mV and a polydispersity index of less than 0.5 indicate the stable and efficient dispersion of cellulose nanomaterials in water. Furthermore, the cellulose nanomaterials exhibited a crystallinity index below 70%, suggesting that the nanomaterials were present in the form of cellulose nanofibers. The XRD and FTIR characterization further revealed that the cellulose nanomaterials possessed the structure of type I cellulose. The structural characteristics of cellulose nanofibers were verified using scanning electron microscopy. In conclusion, the developed method successfully enables the manufacturing of high-quality cellulose nanomaterials. This study contributes substantially to the realization of sustainable conversion technologies for agricultural waste and supports the move towards a circular bioeconomy.