The use of eco-friendly methods for the synthesis of metallic nanoparticles has attracted considerable attention due to their reduced environmental impact and chemical toxicity. Given the rapid rise of antibiotic resistance and the hazards associated with conventional chemical methods, this study aimed to synthesise silver nanoparticles using castor oil as the sole reducing and stabilising agent, and to investigate their antibacterial and antioxidant properties against E. coli and Shigella spp. The nanoparticles obtained were characterised by Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). FTIR results confirmed the presence of hydroxyl, amide, and carboxyl functional groups, which play key roles in the reduction and stabilisation processes. The XRD pattern showed characteristic peaks at 2θ values of 38.1°, 44.3°, 64.5°, and 77.4°, corresponding to the face-centred cubic structure of silver, with an average crystallite size of approximately 16.8 nm. The results demonstrated significant inhibitory and bactericidal effects, with minimum inhibitory concentration (MIC) values of 156.25 µg/mL for both strains and minimum bactericidal concentration (MBC) values of 625 µg/mL for E. coli and 312.5 µg/mL for Shigella spp. In the antioxidant assay, the nanoparticles scavenged 81% of DPPH free radicals at a concentration of 500 µg/mL, with an IC₅₀ value of 41.75 µg/mL. The novelty of this research is the optimisation of synthesis conditions (0.1 mM silver nitrate, 1:10 castor oil-to-precursor ratio, 80 °C, and 2-hour reaction time), which have not previously been reported with castor oil. This optimisation produced the smallest crystallite size and the lowest IC₅₀ value recorded using castor oil. The findings establish castor oil as a highly efficient and cost-effective biological agent for the industrial production of silver nanoparticles with superior bioactivity, providing a promising alternative to conventional antibiotics and synthetic antioxidants in the pharmaceutical and food industries.