Silver nanoparticle (AgNPs) production with antibacterial and antitumor properties is an important application in the medical field. This study introduces a novel organism that can be used for the large-scale production of AgNPs. The edible brown alga Eisenia bicyclis was used as a reducing agent to biosynthesize stable AgNPs. In this study, we achieved producing 50 mg AgNPs using only 1 g dried E. bicyclis seaweed. AgNP biosynthesis was performed at optimized conditions of a reaction temperature of 90°C, a seaweed extract concentration of 0.4%, and an AgNO3 concentration of 0.5 mM within 20 min, and the results showed that the formed nanoparticles are spherical and monodispersed with an average size 18.5 ± 1.2 nm. The antibacterial activity of biosynthesized AgNPs was evaluated against some human clinical pathogens. Results showed that AgNPs had antibacterial activity against all tested bacterial strains, with the appearance of a clear zone equal to or larger than positive controls. Also, there was a concentration-dependent growth inhibition of in vitro cultured breast cancer cells treated with AgNPs and overexpression of p53 and Bax, and underexpression of Bcl-2. AgNPs synthesized by this method provide a potential source for antibacterial and anticancer applications.