The digital transformation in e-commerce has fundamentally reshaped consumer behavior, making social media marketing, brand image, and trust critical drivers of purchasing actions. This study investigates the direct and indirect influences of Social Media Marketing and Brand Image on Purchase Decisions, with Customer Trust acting as a mediating variable, focusing on Shopee users. Utilizing a quantitative approach, empirical data were collected through an online structured questionnaire administered to active Shopee consumers who engage with the platform’s social media channels. The analysis was conducted using Partial Least Squares Structural Equation Modeling (PLS-SEM) to evaluate both the measurement and structural models. The findings reveal that both Social Media Marketing and Brand Image exert a significant positive direct effect on Customer Trust and Purchase Decisions. Furthermore, Customer Trust was confirmed to significantly mediate the relationship between marketing efforts, brand perception, and final purchasing behavior. These empirical results emphasize that while marketing exposure and strong brand identity draw consumer interest, sustainable transaction completion heavily relies on cultivating trust within the digital ecosystem. Practically, the study provides strategic insights for e-commerce platforms to align interactive marketing with rigorous trust-building mechanisms.