In this study, biological activities of silver nanoparticles (AgNP) synthesized by green synthesis method using the fruit extract of Gilaburu (Viburnum opulus L.) plant were investigated. The characterization of the synthesized AgNPs was performed by ultraviolet visible region spectroscopy (UV-Vis), Fourier transform - infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The crystal size was found to be 52.32 nm when the XRD data were calculated with the Debye-Scherrer's equation. The antimicrobial activities of extract and AgNPs were investigated by microdilution and disk diffusion methods. Antibiofilm activities were examined by crystal violet technique. The cytotoxic effects of the extract and AgNPs against MCF-7 (human breast cancer) and HUVEC (human umbilical vein endothelial cells) cell lines were evaluated by MTT assay. The IC50 values for the HUVEC line were found to be 970 µg/mL (AgNP) and 85.24 mg/mL (extract), while the IC50 values for the MCF-7 line were determined as 22 ng/mL (AgNP) and 0.021 µg/mL (extract). To our knowledge, this is the first report to comprehensively examine the biological activities of V. opulus L. extract and biosynthetic AgNPs.