This project investigates how audiences perceive the authenticity of social media influencers as they become increasingly professionalized, especially with the support of agencies, platforms, and other stakeholders. The research unfolds in three phases. First, a qualitative study explores how multi-channel network affiliation disclosure is interpreted by audiences and how such cue of influencer professionalization shapes audience perceptions of influencer authenticity, credibility, and persuasion. Building on the findings, the second phase conceptualizes and operationalizes the multi-dimensional construct of audience-perceived influencer authenticity. Finally, experimental studies examine how different signals of influencer professionalization influence authenticity judgement and audience responses to branded content. This project contributes to influencer marketing by offering conceptual clarity and empirical tools for understanding influencer authenticity in the evolving digital landscape.