In this work we report an easy and simple green synthesis method for gold nanoparticles (GNPs) using willow tree bark extract. Willow tree bark includes aspirin, which acts as a reducing agent. In the present method, there is no need to add any stabilizing agent such as a surfactant to stabilize the synthesized GNPs. Therefore, as an alternative to chemical synthesis, the green synthesis of GNPs with willow tree bark extract is beneficial for its potential biological and medical applications. The optimum conditions for GNP synthesis were obtained by studying the pH and the amount of willow tree bark extract solution. The characteristics and morphologies of GNPs were investigated by UV-Vis spectroscopy and transmission electron microscopy. The interaction between the synthesized GNPs and cysteine was introduced as a new and high-potential colourimetric sensor for the selective recognition and monitoring of cysteine, among other amino acids. The sensitivity and selectivity of GNPs toward cysteine in comparison to other amino acids were studied.