Young climate activist Greta Thunberg was Time magazine’s Person of the Year for 2019. In this study, the authors examine the role of social media in Thunberg’s rise to prominence by investigating Twitter communications over 14 months, analyzing tweet contents and user profiles. The changes in sentiment revealed on Twitter are examined, along with four major events from the period. The paper also compares Swedish and American user characteristics in relation to Thunberg. The findings reveal that celebrities and public figures played an essential role in amplifying the influence of the activist. To categorize the influencer group, the authors identified each user’s occupation, organization, location and position on other social issues. The results show that some people held a negative attitude toward Thunberg, not only because of their skepticism about climate change but also because of their views on other social issues, such as gender and age.