Influencers can effectively promote products and brands but are also leading personalities who might inspire others to support polarizing and/or prosocial issues (e.g., against gender‐based violence, social inequality, and racism). This research analyses the impact of influencer activism on perceived authenticity and prosocial behaviors, focusing on collaborations with brands and nonprofit organizations. Drawing on social influence theory and signaling theory, two key factors are examined: the influencer‐partner congruity, and the influencer‐sociopolitical issue alignment. The research consists of a preliminary study, four experimental studies on behavioral intentions, two experimental studies on proxies of actual behavior, and a content analysis on Instagram comments. The results suggest that congruity and alignment significantly enhance perceived authenticity, which positively impacts attitudes toward the influencer and increases the intention to engage in prosocial behaviors. The article offers further insight into individual engagement in actual prosocial behavior, such as seeking information on an issue, subscribing to newsletters, and signing petitions. The research underscores the importance of selecting congruent influencers, whose values and advocacy efforts are consistent with the brand's or organization's characteristics, and aligned with relevant issues, to enhance perceived authenticity, foster genuine engagement and effectively drive prosocial behaviors through effective partnerships.