jats:titleAbstract</jats:title>jats:pDespite Greta Thunberg's popularity, research has yet to investigate her impact on the public's willingness to take collective action on climate change. Using cross‐sectional data from a nationally representative survey of U.S. adults (jats:italicN</jats:italic> = 1,303), we investigate the “Greta Thunberg Effect,jats:italic”</jats:italic> or whether exposure to Greta Thunberg predicts collective efficacy and intentions to engage in collective action. We find that those who are more familiar with Greta Thunberg have higher intentions of taking collective actions to reduce global warming and that stronger collective efficacy beliefs mediate this relationship. This association between familiarity with Greta Thunberg, collective efficacy beliefs, and collective action intentions is present even after accounting for respondents’ overall support for climate activism. Moderated mediation models testing age and political ideology as moderators of the “Greta Thunberg Effect” indicate that although the indirect effect of familiarity with Greta Thunberg via collective efficacy is present across all age‐groups, and across the political spectrum, it may be stronger among those who identify as more liberal (than conservative). Our findings suggest that young public figures like Greta Thunberg may motivate collective action across the U.S. public, but their effect may be stronger among those with a shared political ideology. Implications for future research and for broadening climate activists’ appeals across the political spectrum are discussed.</jats:p>