The diamond spider, which has not been seen since 1969, was found by volunteers working at Clumber Park near Worksop, north Nottinghamshire.
The two rangers spotted a 7mm-long specimen of 'Thanatus formicinus' while searching for arachnids in an area of heathland for an ecological survey.
It has only ever been recorded in the UK on three occasions, all of them in the south of England.
Volunteer Lucy Stockton said:
“The spider ran away from me twice but with persistence and some luck I caught it; at the time I had no idea that it would turn out to be such a rare find. Upon closer inspection our spider had a conspicuous ‘cardiac mark’, a black diamond shape on its abdomen, edged with white that helped us to identify it.
“We were thrilled to have discovered this new resident of Clumber Park and to prove that this species is definitely not extinct in the UK.”
The last recorded sightings of the diamond spider occurred in Legsheath and Duddleswell, in Ashdown Forest, in 1969. The arachnid was also found near Brockenhurst, in the New Forest, at the end of the 19th century. Its habitat includes boggy areas with moss, purple moor grass and heather. Its English name derives from the thin black diamond on its back.
The astonishing find has stunned preservation group Buglife. Chief executive Mark Shardlow said:
“We are absolutely delighted that this pretty little spider has been found, we had almost given up hope. It is a testament to the crucial importance of charities like the National Trust saving and managing heathland habitats.”