Surrey Wildlife Trust has introduced 30 nationally-endangered water voles to a nature reserve near Elstead.
The release on the River Wey comes following a "successful" release on the upper river.
The water voles are being introduced downstream on 10 sites between Frensham and Elstead, the wildlife trust said.
James Herd, director of reserves management at the trust, said: "As we work towards the restoration of natural ecosystems and landscapes county wide, I hope this release is one of many reintroductions of native species that we'll see in the years ahead."
Water voles have been functionally extinct in Surrey since the early 2000s, but co-ordinated reintroduction and habitat improvement efforts by nature restoration groups are working to help the endearing species thrive again.
The trust said in a statement that it hoped that over the coming months, the aquatic rodents will "adapt easily to life on the 16-hectare reserve at Thundry Meadows and establish widely across the top of the Wey catchment".
Their return would also benefit other species which depend on healthy wetland habitats including herons and birds of prey, they added.
David Elliott, National Trust lead ranger for the South Downs West said: "Nature wins when we work together."
"It's so encouraging to see the water voles returning after all this time, they have been locally extinct for decades.
"The return of this enchanting species adds a missing piece to this jigsaw."
The release is part of a wider effort led by the National Trust, River Wey Trust and South East Rivers Trust to reintroduce the locally extinct species back to the River Wey.
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