Influencer marketing research is rapidly evolving, with growing interest in the dynamics between social media influencers and their followers. Engagement, defined as the active interaction between followers and influencers, remains a fragmented area of study, despite valuable insights from recent research. This systematic literature review synthesizes findings from 43 scholarly articles retrieved from the Web of Science database, focusing explicitly on engagement with SMIs. Using the Antecedents, Decisions, Outcomes (ADO) framework, the review categorizes factors influencing engagement into source, content, and audience-related dimensions and integrates them into a comprehensive framework. This framework clarifies how these factors drive various engagement behaviors, such as likes, comments, and shares, and explores conceptualizations of engagement across cognitive, emotional, and behavioral dimensions. By consolidating existing knowledge, identifying gaps, and proposing future research directions, this review enhances theoretical understanding and offers actionable insights for practitioners aiming to increase engagement with social media influencers.