Silver nanoparticles (AgNPs) are one of the best tools for cancer treatment due to their unique physical and chemical properties. Although chemical methods for the synthesis of silver nanoparticles are effective, but they are generally expensive and may carrying traces of toxic substances, which are hazardous for human and environment. For this reason, in recent years, researchers have tried to develop biological methods for nanoparticles synthesize using living organisms or their products, which are environmental friendly and harmless for humans. In this study, cell-free extract of Polysiphonia algae used as reducing agent for green synthesis of AgNPs. The colorless AgNO3/extract solution changed to dark brown after 60 minutes that indicating the formation of the AgNPs. The progress of the reaction measured by UV-visible spectroscopy showed the maximum absorption peak at ~420 nm wavelength that confirmed the synthesis of silver nanoparticles. The scanning electron microscope (SEM) and transmission electron microscope (TEM) micrograph showed the most of biosynthesized silver nanoparticles have predominately-spherical structure with the size range of 5 to 25 nm that are suitable for penetration into cancer cells. Fourier Transform Infrared Spectroscopy (FTIR) analysis of algal extract was used to identify the functional groups responsible for the reduction and stabilization of silver nanoparticles and confirmed the presence of phenolic, aldehyde/ketone, alcoholic, and amine compounds in extract. To investigate the anticancer effects of silver nanoparticles on MCF-7 cell lines, cell survival and proliferation were measured by MTT and flow cytometry and the results confirmed the anti-proliferation potential of AgNPs against breast cancer cells. In conclusion, the formation of silver nanoparticles with anticancer effect only 60 minutes after the start of the reaction in the presence of very low concentration of algal extract and at ambient temperature indicated that Pol
