In their marketing efforts, companies increasingly abandon traditional celebrity endorsers in favor of social media influencers, such as vloggers and Instafamous personalities. The effectiveness of using influencer endorsements as compared to traditional celebrity endorsements is not well understood. Therefore, the present research investigated the impact of celebrity vs. influencer endorsements on advertising effectiveness (attitudes toward the advertisement and product, and purchase intention), moderated by product-endorser fit. Moreover, this research investigated two potential mediators underlying this relationship: identification (perceived similarity and wishful identification) and credibility (trustworthiness and expertise). Two experiments (N = 131, N = 446) investigated celebrity vs. influencer endorsers with good vs. poor fit with a beauty and a fitness product (Study 1), or a food and a fashion product (Study 2). Overall, our results showed that participants identify more with influencers than celebrities, feel more similar to influencers than celebrities, and trust influencers more than celebrities. In terms of advertising effectiveness, similarity, wishful identification, and trust mediate the relationship between type of endorser and advertising effectiveness. Product-endorser did not explain the relationship between type of endorser and any of the mediating and dependent variables. In all, our results show the added value of using influencer endorsers over celebrity endorsers and the importance of similarity, identification and trust in this process.