This chapter explores youth climate activism as a site of prefigurative politics through the lens of Hannah Arendt’s political philosophy. It argues that young activists do not merely protest current injustices but actively embody and prefigure the sustainable, democratic futures they envision. Drawing on Arendt’s concepts of natality, political action, and plurality, the chapter frames youth-led movements such as Fridays for Future and Extinction Rebellion as generative political forces that transform public spaces, resist systemic inertia, and experiment with participatory governance. Rather than treating utopia as a distant blueprint, these movements enact concrete, evolving alternatives in the present, aligning with Arendt’s vision of politics as dynamic, relational, and world-building. Prefigurative politics is thus recast not just as a strategic practice but as an ontological act of collective becoming. Through councils, civil disobedience, and sustainable everyday practices, young activists cultivate spaces of appearance where freedom, solidarity, and new political possibilities can emerge. The chapter ultimately suggests that youth climate activism represents a form of Arendtian praxis: a hopeful, plural, and participatory politics that responds to the existential crises of our time by creating new beginnings in the here and now.