The diamond spider, so-called due to its markings, was presumed extinct, but was found by two trust volunteers on heath land at Clumber Park, Worksop.
The spider, 7mm (0.2 inch), has only been recorded three times, all in the south of England, and not since 1969.
Lucy Stockton, who made the discovery, said she was "thrilled" by the find and that the species still existed.