A campaign to raise £100 million is underway to renature 13,000 hectares of land on the South Downs.
The area would be the size of 21,000 football pitches and would be a habitat for plants and animals to thrive in.
The extra land would mean 33% of the national park is managed for nature, which exceeds a UN-backed target of 30% by 2030.
The biodiversity of the park includes more than 20 species of butterfly along with 12 native reptile and amphibian species.
The project comes as the latest national State of Nature report showed that 41% of UK species studied have declined with 133 species assessed to have already been lost from British shores since 1500.
Around a quarter of the UK's mammals could also be at risk of disappearing altogether with climate change, pollution and habitat loss being among the causing factors.
Ecologist Andrew Lee, who heads countryside policy and management for the national park, said:
"Nature needs us now and we also need nature, perhaps now more than ever before in this post-pandemic world where green spaces have taken on a new level of importance.
"Apart from being incredibly beautiful and part of our shared appreciation for planet Earth, nature gives us everything - whether it be clean water, fresh air or food to eat.
"We're launching this campaign without a moment to lose because it's time for all of us to help nature to renature."
It is hoped that the rewilding will be achieved through a series of projects, which will the National Park work with farmers, land managers, communities and local authorities.
It will include "everything" from hedgerow restoration, planting thousands of trees, restoring individual village ponds and planting new wildflower corridors.