Since late 2018, young people around the world have united to demand greater action on climate change. Aside from their stated concerns and demands, however, very little is known about why young people have been joining this growing movement. Using a large sample (N = 4057) of people in Switzerland aged between 14 and 25, we show that social identity is most strongly associated with participation, followed by beliefs about the effectiveness of youth strikes, level of education, and worry about climate change. Our findings affirm the relevance of both climate change risk perceptions and social identity-related processes for collective climate change action, and pave the way for promising opportunities in theory development and integration. The study also provides lessons for those who seek to maintain and increase collective action on climate change: concern about climate change is an important motivating factor, but social identity processes are at least as relevant for young people’s participation.