This study uses the Stimulus-Organism-Response (SOR) model to investigate how Social Media Influencer (SMI) attributes impact Generation Z’s purchase intention and behaviour regarding counterfeit fashion goods in Malaysia. This study also explores the mediating role of self-determination in connecting SMI attributes to purchase intention. This study was based on 348 survey responses from Malaysian Generation Z aged 18–24. The findings indicate a positive correlation between purchase intention and behaviour, with SMI attributes significantly affecting purchase intention. Specifically, ideality mediates the link between visual aesthetics and purchase intention, while relatedness mediates the connection between similarity and purchase intention, drawing on self-determination theory (SDT). This research contributes insights into the unique traits of SMIs and their influence on Generation Z’s purchasing behaviour, emphasising the mediating role of SDT dimensions.