BACKGROUND: Various physical and chemical methods for synthesis of metal nanoparticles have had some drawbacks. Therefore, green synthesis of gold nanoparticles (Au NPs) has became one of the most crucial emerging areas of nanobiotechnology. In the present study, plant-mediated synthesis of Au NPs was performed using Djulis (Chenopodium formosanum) shell extract as a reducing and stabilizing agent. RESULTS: Reaction parameters were manipulated to optimize the Au NPs using a UV-visible spectrophotometer. Optimized Au NPs with a surface plasmon resonance band at 533 nm were prepared using a 744 µg mL extract and a solution of pH 2.62 chloroauric acid (HAuCl ·3H O) at 40 °C. High-resolution transmission electron microscopy (HR-TEM) results indicated that most of the resultant Au NPs were spherical in shape and exhibited a mean size of 8 ± 6 nm. Energy-dispersive X-ray spectroscopy (EDS), and selected area electron diffraction (SAED), and X-ray diffraction (XRD) confirmed the elemental gold and crystalline nature of the resultant NPs. FTIR spectrum analysis indicated the critical role of phenolic groups in the reduction of Au ions and stabilization of the formed Au NPs. Moreover, the synthesized Au NPs possessed antibacterial activity for Escherichia coli and Staphylococcus aureus. CONCLUSION: In this study, Au NPs were synthesized with high efficiency and stability using Djulis shell extract, and related antibacterial applications were demonstrated. © 2019 Society of Chemical Industry.