A coastal management scheme has been completed at Steart Marshes near Bridgwater.
A sea wall has been breached to allow sea water to flood a newly cut channel to create a 300 hectare (740 acre) saltmarsh nature reserve.
The £21m scheme will help protect homes and businesses from flooding due to climate change and rising sea levels.
The new habitat is owned by the Environment Agency and is managed by the Wildfowl and Wetlands Trust (WWT).
The trust said it was the UK's largest coastal management scheme.
Peter Morris, from the trust, said: "Climate change and extreme weather events are becoming more of an issue.
"What we're proving here is that we can deal with these issues on the coast by working with nature rather than against it, rather than building concrete walls which eventually get higher and higher and are not viable."
Over the past five years, new embankments have been built about 1km (0.6 miles) inland to allow the area to flood with saltwater.
The first extremely high tide, which will allow the reserve to flood fully for the first time, is expected to occur next week.
The saltmarsh will provide much-needed habitat for wading birds and wildfowl, and for rare water voles and great crested newts.