Palaeontologist Dr David Penney, of the University of Manchester, found the 4cm long by 2cm wide fossil during a visit to a museum in the Dominican Republic.
Since the discovery two years ago, he has used droplets of blood in the amber to reveal the age of the specimen.
It is thought to be the first time spider blood has been found in amber and scientists hope to extract its DNA.
Dr Penney, of the School of Earth, Atmospheric and Environmental Sciences, said he had used the blood droplets to trace how, when and where the spider died.
Dr Penney believes it was climbing up a tree 20 million years ago when it was hit on the head by fast flowing resin, became engulfed in the resin and died.