This study explored how the effectiveness of online advertising impacts purchase decisions, with a focus on the mediating role of customer trust. Employing a quantitative methodology with a cross-sectional experimental design, the research was conducted at the Catholic University of Santa Maria in Arequipa, Peru, involving 300 student participants. The instrument’s validity was confirmed through exploratory and confirmatory factor analyses. We utilized partial least squares structural equation modeling to assess both direct and indirect effects. Findings indicate that customer trust significantly influences online purchase decisions, and that the perceived usefulness of online advertising substantially affects both customer trust and purchase decisions. The study concludes that both customer trust and the usefulness of online advertising are crucial predictors of online purchasing behavior. Future research should aim at enhancing the perceived value of online advertisements and fostering customer trust to maximize advertising effectiveness and stimulate online shopping.