In this present study, marine red algae Hypnea valentiae capped silver nanoparticles (Hv-AgNPs) have been prepared through the green method and characterized by various analytical tools. The prepared Hv-AgNPs were confirmed by Ultraviolet-visible (UV) spectroscopy at 430 nm. The X-ray diffraction (XRD) analysis affirmed the face-centered cubic crystalline structure of Hv-AgNPs. Transmission electron microscope (TEM) revealed the Hv-AgNPs shape as spherical with an average size of 10–45 nm. Fourier Transform Infrared (FR-IR) analysis depicted the presence of various biomolecules such as carboxyl, phenolic, and amine groups, which acted as reducing and capping agents of biosynthesized silver nanoparticles. The Hv-AgNPs had significant antibacterial activity against four bacterial pathogens. The 2,2’-Diphenyl-1-picrylhydrazy (DPPH) assay revealed the strong free radical scavenging ability of Hv-AgNPs. The Hv-AgNPs exhibited excellent anti-cancer activity against HT-29 human colon cancer (IC50–24.6 μg/ml) and A549 lung cancer cell lines (IC50–5.917 µg/ml) by MTT assay. Furthermore, the investigations demonstrated that green synthesized silver nanoparticles possess high anti-bacterial, anti-oxidant and anti-cancer activities.