The young Swede says in a video posted on Instagram that the award was “more money than I can even begin to imagine”.
Greta Thunberg has turned down the chance to become a millionaire and will instead donate a €1m (£902,000) prize to environmental causes.
Ms Thunberg, 17, won the inaugural Gulbenkian Prize for Humanity, awarded by a Portuguese philanthropic organisation.
The climate change campaigner, who has inspired protests and activism across the world, said her foundation would distribute the money “as quickly as possible”.
The teenager said on Instagram that the prize would help “support organisations and projects that are fighting for a sustainable world, defending nature and supporting people already facing the worst impacts of the climate – and ecological crisis”.
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The first to get some of the cash – €100,000 (£92,000) each – are SOS Amazonia – which provides medical aid for indigenous communities battling coronavirus – and the Stop Ecocide Foundation, a campaign to make “ecocide” an international crime.
The Swede said that the prize was “more money than I can even begin to imagine” and she hoped it would help her “do more good in the world”.
“All the prize money will be donated through my foundation to different organisations and projects who are working to help people on the frontlines affected by the climate crisis and ecological crisis, especially in the Global South,” she added.
Thunberg was selected from 136 nominees in 46 countries for the prize, which recognises people and groups who have made important contributions in helping to mitigate and adapt to climate change.
Jorge Sampaio, chair of the prize jury and former president of Portugal, called Thunberg “one of the most remarkable figures of our days” for her ability to mobilise younger generations in support of her cause.
Friday was the 100th week of the student climate protests that Thunberg started in mid-2018 by holding a solitary vigil outside the Swedish parliament.
Since then, millions of children have followed her lead and skipped classes to demand action on climate change.
Thunberg has also addressed the United Nations on the issue and met politicians and leaders across the world.