A novel solid catalyst with basic properties was developed from untreated smectite chemically modified by treatment with potassium fluoride, resulting in a material with excellent stability and high catalytic activity in the conversion of soybean oil into biodiesel in the presence of methyl alcohol. The percentage of conversion into methyl esters of 99.7% was obtained under conditions of 1 h of reaction time at 338 K, 15 wt.% catalyst and molar ratio of methanol/oil of 6:1. A concomitant study was made to evaluate the effect of the mineralogical composition of the untreated clay on the properties of the catalysts and their performance in the transesterification reaction.