Baby spiders were introduced on RSPB land at Strumpshaw Fen on the Norfolk and Suffolk Broads in 2012.
Latest figures show the number of nursery webs counted between July and October has risen from 184 in 2014 to 480 in 2015 - a 160% increase.
The spiders, which grow up to 7cm long, can deliver a painful bite but are not considered dangerous.
"This is a species that is clearly able to thrive in the Broadland grazing marshes that have been so carefully restored by conservation organisations in recent years," said Helen Smith, an ecologist contracted by Natural England.
"It's already made a great start on colonising the area's extensive ditch networks and, in the process, has taken a big step back from the brink."
Tim Strudwick, RSPB site manager at Strumpshaw Fen nature reserve, said: "It is fantastic to see the spiders now thriving on the reserve, having been first released in 2012.
"The spiders are doing so well due to the excellent condition of the habitat and our management of the grazing marshes is maintaining ideal conditions for them."
Up until 2010 there were only three known populations in the UK of the spider, leaving the species very vulnerable and at real risk of extinction.
They primarily feed on aquatic invertebrates such as pond skaters and dragonflies.
Work to boost a community of rare fen raft spiders in Norfolk has proved a success.