Instagram has emerged as a critical platform that influences consumer behavior, as a result of the rapid expansion of social media. This has revolutionized marketing communication. The functions of perceived authenticity and brand trust are investigated in this study using the Stimulus–Organism–Response (S–O–R) framework. A sequential mixed-methods design was implemented. Semi-structured interviews (n=32) were employed in Study 1 to identify key themes, while structural equation modeling was employed in Study 2 to validate the model using survey data (n=415). The results suggest that Instagram content has a substantial impact on the perception of authenticity, which in turn enhances brand trust and influences purchase decisions. The findings substantiate the mediating role of trust and authenticity, emphasizing that the efficacy of marketing is contingent upon credibility rather than mere exposure. The research expands the S–O–R framework to social media marketing and provides managerial insights for the development of authentic, trust-building content strategies that influence consumer decision-making