Green nanomaterial production techniques are desired for medicinal applications because of their biocompatibility and lack of toxic by‐products. This work reports the biological synthesis of silver nanoparticles (AgNPs) from silver nitrate solutions using the cell‐free extract of Polysiphonia alga as a reducing agent. The visible yellow‐brown colour formation and surface plasmon resonance at 420 nm by ultraviolet–visible spectroscopy indicates the reduction of silver nitrate and biosynthesis of AgNPs. For more characterisation of green synthesised AgNPs, Fourier‐transform infrared (FTIR) spectroscopy analysis is used and the results confirmed the presence of functional groups capping in the AgNPs. The scanning electron microscopy and transmission electron microscopy images showed mostly spherical AgNPs with a size range between 5 and 25 nm and the concentration of 3.1 × 10 ⁶ particles/ml. The results of energy‐dispersive X‐ray analysis confirmedthe presence of Ag in the synthesised nanoparticles. The MTT assay and flow cytometry are used to determine the toxicity of AgNPs against breast cancer MCF‐7 cell line and the results showed the best inhibitory activity at 100 µg/ml. Generally, green synthesis of anticancer AgNPs using filamentous red alga Polysiphonia extract can easily be scaled up for many biomedical applications such as defence against the cancerous cells.

Biodiesel Production from Azolla filiculoides (Water Fern)