In the current study, using Cleistocalyx operculatus bud extracts to synthesize silver nanoparticles is considered a novel, eco-friendly, and low-priced process that advances chemical and physical methods. C. operculatus bud extracts played a dual part in reducing and stabilizing silver nanoparticles. The formation of silver nanoparticles was recognized on UV–Vis spectra at an absorption maximum of 432 nm. Transmission electron microscopy images detected the quasi-spherical nanoparticle shape with an average size of 26.2 nm corresponding with water extract and 27.5 nm synthesized by ethanol–water extract. The obtained silver nanoparticles had moderate stability with zeta potential ranging from - 15.4 to - 38.4 mV. The phase purity of the bio-synthesized materials was confirmed by an X-ray diffraction pattern. The Fourier transform infrared analysis demonstrated hydroxyl groups acting as stabilizing agents in the extract. Silver nanoparticles synthesized from ethanol–water extract exhibited higher antibacterial and anticancer activity than nanomaterials prepared with water extract. Positive-gram bacteria (Bacillus subtilis, Lactobacillus fermentum, and Staphylococcus aureus, negative-gram bacteria (Escherichia coli, Pseudomonas aeruginosa, and Salmonella enterica), and cancer cell lines (lung cancer A549, hepatic cancer Hep-G2, epidermal carcinoma KB, and breast cancer MCF-7) were both inhibited by the nanoparticles synthesized with ethanol–water extract.