This study explores the influence of sustainability-promoting social media influencers on their followers’ adoption of sustainable behaviours. Specifically, it examines how influencer credibility (IC), perceived behavioral control (PBC), and attitudes toward sustainability (AS) shape actual sustainable behavior (ASB) among followers. Using Partial Least Squares Structural Equation Modelling (PLS-SEM) based on the survey distributed among the followers of sustainability-promoting influencers in the winter of 2024–2025, our findings reveal that influencer credibility is a key driver in advancing positive attitudes toward sustainability, as followers who perceive an influencer as knowledgeable, trustworthy, and relatable are more likely to develop favorable views on sustainable practices. Furthermore, influencer credibility has a direct impact on the actual sustainable behaviors of their followers. Perceived behavioral control is also a significant predictor of both attitudes toward sustainability and sustainable behavior. However, positive attitudes toward sustainability alone may not directly drive behavioral change without sufficient perceived control and credible influence. These findings highlight the importance of influencers maintaining high credibility and empowering followers with practical, achievable guidance to facilitate sustainable lifestyle changes. Thus, this study highlights the potential for social media influencers to play a meaningful role in advancing sustainability.