The new reserve is the second dark sky reserve in England, along with Exmoor national park.
It also joins two national parks in Wales - the Brecon Beacons and Snowdonia - as among just 11 areas worldwide to win status as international dark sky reserves.
Reserves are designated by the international body if they meet standards for night sky quality and natural darkness in a core area, with a peripheral area that supports preserving the dark sky at the core.
The designation comes after national park ranger Dan Oakley and volunteers spent three years mapping the quality of night skies across the park, which has 2 million people living in or within three miles (5km) of its boundaries.
Local authorities have also worked to replace 2,700 street lamps across the national park with LED lights which direct light downwards and reduce light pollution.
The move to create a dark sky reserve was backed by more than 70 parish, town and county councils and other organisations and more then 1,300 people signed a “dark skies pledge”.
In the future, the South Downs National Park Authority will use its role as a planning authority to protect the skies above the park as well as the landscape on the ground.
Towns and cities on the edge of the park, which stretches through parts of Hampshire, West and East Sussex, include Portsmouth, Brighton and Hove, Eastbourne, Winchester and Chichester.
Oakley said: “With the south of England under threat from losing its last few patches of properly dark skies this is a statement that the skies of the South Downs are worth protecting.
“With 2 million people living within 5km of the national park, the reserve will be one of the most accessible in the world and certainly one of the most cared for.”
The International Dark-Sky Association’s executive director, J Scott Feierabend, announced the designation.
He said: “It is remarkable that a true dark-sky experience remains within reach of nearly 17 million people in Greater London and south-east England, and a testament to the hard work of South Downs staff and area residents in keeping it that way.”
Rural affairs minister Rory Stewart hailed the project, saying: “The South Downs national park is bringing communities not just an opportunity to see the English countryside at its finest, but also stars far beyond our planet.”
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