In this study, silver (Ag) nanoparticles were successfully biosynthesized from AgNO3 through a simple green route using the lichen extract [Cetraria islandica (L) Ach] as a reducing and stabilizing agent. The mean sizes of spherical Ag nanoparticles with diameters ranging from 5 nm to 29 nm were obtained at different reaction conditions. The nanoparticles were characterized using transmission electron microscopy (TEM), energy dispersive X-ray (EDX), ultraviolet-visible (UV-VIS) and Fourier transform infrared spectroscopy (FTIR) techniques. Response surface methodology (RSM) was used to investigate the effects of temperature, reaction time and AgNO3/lichen ratio on green synthesis of Ag nanoparticles. The results showed that the increase in reaction time and AgNO3/lichen ratio caused a decrease of particle size and the increase in temperature resulted in bigger particles.