Social Media Influencers (SMIs) and their sponsored posts on the Social Media represent a growing part of tomorrow’s digital marketing landscape. Marketers hire them to recommend their products to their large audiences through friendly and engaging social media posts. To please marketers, increasing and maintaining their audience size is of the utmost importance. Consequently, number of followers, likes, and comments have become the metrics of SMIs success. However, recent contradictory publications of business magazines and academic research raised the question of SMIs’ online popularity, credibility, and their efficiency as brand endorsers. Hence, an online-based survey experiment based examined the audience’s attitude of SMI credibility across three different level of online popularity using Ohanian’s (1990) source credibility model. Further, the relationship between SMI credibility and brand attitude, and the mediating effect of attitude toward sponsored post were investigated using a research model adapted from Attitude toward advertising models. The findings revealed that the more an SMI is popular, the more he is perceived as a credible brand advocate, and SMI credibility was found to positively and directly influences brand attitude. Semi-structured interviews strengthened the discussion of the quantitative results. Theoretical and managerial contributions of the study are presented and suggestions for future research ensue.