The rise in celebrity athletes’ influence due to social media has had a massive effect on endorsement opportunities. Social media has given celebrity athletes an owned platform from which to leverage their audience into business opportunities, but an understanding of how authenticity affects the consumer was needed. Therefore, the purpose of this study was to determine how the perceived authenticity of celebrity athletes’ social media endorsement posts affects brand attitudes and purchase intentions. Two experiments were conducted using fictitious Twitter posts including celebrity athletes to manipulate authenticity. Study 1 established that authenticity of a post has a significant effect on brand attitudes and purchase intention. Study 2 introduced two new celebrities and one new product. The importance of authenticity remained consistent with Study 1. Additionally, the type of brand had a greater effect when the promoted product required more purchase consideration when compared to an impulse purchase. Lastly, the celebrity promoting the product was not found to be significant. This research expands on the growing stream of knowledge related to authenticity in marketing communications and confirms its vital role.