Head of conservation and land at Manx Wildlife Trust, David Bellamy, said the reserve was "one of the very best sites in the whole island to experience and enjoy Manx nature at its very best".
He said the new designation would provide the greatest level of protection available under Manx law and was a fitting way to mark the King's visit on Tuesday, given his longstanding interest in nature conservation.
The reserve covers 7.16 hectares (17.69 acres), with habitats including sand dunes, grassland, reedbeds and foreshore. It is also home to rare plants, including the pyramidal orchid, and supports beach-nesting birds such as ringed plovers and oystercatchers.
