Due to the growing interest of consumers in activist topics and social responsibility, brands are increasingly incorporating these aspects into their strategies. Social media has become a crucial tool for spreading activist messages, yet little research has explored the role of sponsorship in this context. Therefore, this study investigates the impact of sponsorship on the effectiveness of environmental brand activism messages in user-generated content. A quantitative online survey was conducted, and data were analyzed using the Hayes’ PROCESS model. Additional insights were gained through an exploratory analysis. The findings reveal that perceived influencer authenticity mediates the relationship between influencer-cause fit and user engagement, but only in the context of sponsored user-generated content. This suggests that authenticity plays a key role in the effectiveness of sponsored brand activism campaigns. However, sponsorship disclosure does not moderate the relationship between the influencer-cause fit and perceived authenticity. With these findings, the study contributes to Congruence Theory and provides impulses for future research on Persuasion Knowledge Theory by highlighting the effects of sponsorship on engagement. Form a managerial perspective, the results emphasize the importance of carefully selecting cooperation partners to ensure a strong influencer causefit and enhance perceived authenticity. Future research should further emphasize the role of influencer authenticity and user engagement in sponsored environmental brand activism content.