The £12 million scheme should attract several species of birds to the UK including some that have not nested in the country for 400 years, the RSPB said.
Wallasea Island was a wildlife wetland 500 years ago before it was reclaimed for agriculture.
It will be restored by building tidal controls into existing sea walls. Work is due to start in two years once funds and planning permission are secured.
The island's proximity to the Thames Gateway will also provide residents with access to a wild coast, the conservation body said.
"Wallasea Island is only 16km from the Thames Gateway and once the work is done it will be a breathing space for those who live there," maintained project manager Mark Dixon.
Last year, DEFRA breached sea walls on the island's northern edge to create 113ha of wetland that is managed by the RSPB. Faber Maunsell designed the DEFRA project and is working on the RSPB's scheme. ABPmer will conduct the environmental impact assessment.