Secretary of State Hilary Benn joined members of the South Downs Society and other campaigning organisations to celebrate the confirmation of the South Downs as a national park.
Hilary Benn announced in March that he had decided the South Downs should become a national park, but it wasn't until Thursday that he signed the designation order, making the South Downs National Park a reality in law.
The signing and celebrations took place in Ditchling, one of the villages which is now in the national park.
John Songhurst, chairman of the South Downs Society, said, "This is a proud day for the society and all those who have campaigned for the protection of the South Downs.
"The national park means more can be done to protect our precious wildlife and enable it to thrive.
"Landscape features such as prehistoric earthworks and the dewponds will be properly conserved. Villages will benefit too because national parks create jobs and additional income.
"The South Downs has been waiting for more than 60 years for this protection, so there is a great deal to be done and the National Park Authority has its work cut out.
"We will be presenting the authority with suggestions for its priorities over the first months and years and offering the skills and assistance of our members with the tasks ahead."
He said the society would continue to work with the Campaign to Protect Rural England, the Ramblers and various wildlife trusts to help the National Park Authority with the challenges it may face in the coming years.