Hundreds of water voles who live on the DP World London Gateway site in Stanford-le-Hope are to be relocated to a new riverside habitat.
Conservationists are moving the 300 rodents away from Thurrock to the River Colne, near Colchester, 50 miles away.
More than 50,000 animals, including rare species like great-crested newts, have already been moved from the site.
New wetland is being created as part of the building of the port to compensate for habitat lost to the development.
DP World, the developer of the £1.5bn London Gateway port, is planning to meet obligations under EU law by creating new habitat for wildlife.
The company is spending £50m on measures such as turning farmland into new tidal habitat to attract wading and migrating birds by breaching a 975ft (300m) sea wall later this year.
Darren Tansley, from Essex Wildlife Trust, told a reporter that water vole numbers in England had declined by more than 90% in recent years as a result of habitat destruction and attacks by mink.
He added that voles had vanished from the River Colne in the past few years.
Mr Tansley said: “The river has been surveyed for at least two decades and we have watched water voles decline and become extinct mainly because of mink.
“We’ve got the mink down to virtually zero – we have virtually wiped them out from the catchment – and it is now safe for the voles to come back to the river.”
He added that moving a coastal population of water voles inland would also save them from sea level rises and coastal development.
The habitat along the Colne was mostly very good and it was a “perfect water vole river”.