The aqueous fraction of Polyscias scutellaria leaf extract (PSE) has been used as a reducing agent and stabilizer in the green synthesis of gold nanoparticles (AuNPs). UV-Vis spectrophotometry, particle size analyzer (PSA), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy-selected area electron diffraction (TEM-SAED), and X-ray diffraction (XRD) were used to characterize AuNPs. The AuNPs have a size of 5-20 nm and have a face centered cubic (fcc) crystal structure and are stable for 21 days. Phenolic compounds, which are secondary metabolites of PSE, act as an active compound to reduce Au3+ ion to Au0, as well as stabilize the AuNPs through their surface interaction with carbonyl and hydroxyl groups of phenols. AuNPs exhibit excellent catalytic activity for the reduction of methylene blue with NaBH4. The reduction of methylene blue using AuNPs catalysts is a pseudo-first-order reaction with a reduction rate constant (kobs) of 0.0223 min-1.