We have presented a biological and eco-friendly method for the synthesis of gold nanoparticles from gold precursor (HAuCl4) using Sapindus mukorossi Gaertn. fruit pericarp (soapnut shells). We investigate the production of gold nanoparticles as a function of the concentration of HAuCl4 and the amount of soapnut shells. Average nanoparticle sizes of 9, 17, and 19 nm were obtained by using the HAuCl4 concentrations of 1, 5, and 10 mM, respectively, with a fixed amount of soapnut shells extract. The resulted gold nanoparticles are highly crystalline face-centered cubic (fcc) structures. FT–IR analysis suggests that the obtained gold nanoparticles might be stabilized through the interactions of carboxylic groups in the saponins and the carbonyl groups in the flavonoids present in the soapnut shells. These soapnut shells mediated gold nanoparticles were demonstrated to have good catalytic activity for the chemical reduction of p-nitroaniline.