A persistent bottleneck in the deployment of RNA interference (RNAi) technologies in agriculture is the instability of RNA molecules under real-world conditions. RNA is readily degraded by nucleases and environmental factors, limiting its persistence and efficacy. Nanoparticle carriers have emerged as a promising strategy to protect RNA and improve delivery, thereby enhancing the performance of RNAi-based applications. Cellulose nanocrystals (CNCs) are renewable, biodegradable nanoparticles that provide a sustainable alternative to many existing nanomaterials. To date, no studies have investigated CNCs as carriers for small interfering RNA (siRNA) delivery in plants. To address this knowledge gap, polyethyleneimine-functionalised CNCs were synthesised and evaluated for their physicochemical properties, siRNA loading capacity, nuclease protection, plant uptake, and cytotoxicity in human lung cells. Gene silencing was assessed in Nicotiana benthamiana 16C plants expressing green fluorescent protein (GFP). Functionalised CNCs successfully delivered siRNA to plant tissues, reducing GFP expression by approximately 80% after one week with no evidence of phytotoxicity. These findings identify functionalised CNCs as a biodegradable platform for exogenous siRNA delivery and support their development as sustainable RNA carriers for agricultural applications.