In order to meet the current environmental demand in the reduction of greenhouse gas emissions (GHG), and the preservation of water with the reduction of the incorrect disposal of residues, particularly soybean oil after consumption, the objective of this paper is to evaluate the biodiesel obtained from waste cooking oil (WCO) purified using a Brazilian clay after a modified acid treatment, less aggressive to the environment. The clay, before and after the treatment, was characterized by XRD, FTIR, and SEM/EDS. The biodiesel had its appearance, color, kinematic viscosity, density, water content, acidity level, saponification index, and combustion behavior measured. The results indicated that the biodiesel meets the requirements of the Brazilian standard for biodiesels, with the exception of a higher water content. Therefore, the effectiveness of the purification process of the WCO using the modified Brazilian clay was demonstrated.