The adder's-tongue spearwort is only found in two places in the UK - Badgeworth, near Gloucester, and on Inglestone Common, near Bristol.
At Badgeworth, the tiny nature reserve boasts a small crop of the plant but in South Gloucestershire the buttercup is only spotted once every five years.
Plant expert Richard Lansdown said: "We wouldn't normally reintroduce but Inglestone is down to a single plant."
The species, more commonly known as the Badgeworth buttercup, relies on bare, wet ground that has been churned up by cattle to set seed and thrive.
But with fewer animals grazing the common in recent years, it has become overrun with scrub and the buttercup now "only grows intermittently" at a single pond.
"I sieved out the mud and extracted all the seeds [at Inglestone] and didn't find any from the Badgeworth buttercup - there weren't any left at all," said Mr Lansdown.
"So we've had to use seed originally collected from the site some years ago that was sent to the Millennium Seed Bank."
Using the 270 plus seeds held at Kew, 200 plants were propagated and about 80 planted out at Inglestone with a further planting planned for mid-June
"The two sites have been declining for a very, very long time," said Mr Lansdown.
"But we've planted out 83 plants in different locations at Inglestone and at least half of them are surviving and doing well and have begun fruiting which will add to the seed bank."
A rare buttercup is being reintroduced in South Gloucestershire.