Climate change and the concerns it raises for the environment and all those inhabiting planet earth, human and nonhuman alike, have prompted waves of activism since the last decades of the twentieth century. Over the last few years, however, a novel form of activism has emerged, apparently led by children and youth from all over the world. This article studies how one of its most prominent leaders, Greta Thunberg, and her climate activism may be read as a life-writing project. Drawing on traditions of social movements and testimony, rights discourse, rhetoric and strong emotions are strategically deployed to generate affective and effective engagement and action. The self that is in the making in Thunberg's audiovisual and written life-writing texts is arguably a testimonial 'I' to lay bare injustice. Her self-construction hinges upon the denunciation of broader systemic causes than mere lack of attention to the climate crisis.