Paleontologists discovered the sample in the reproductive tract of an ancient female crustacean encased in resin -- one of several samples of ostracods from Myanmar.
The previously unknown species of crustacean, now named Myanmarcypris hui, resembles a modern day mussel and is an example of an ostracod.
Ostracods are small animals that date back some 500 million years, and can still be found in oceans, freshwater lakes and rivers.
Using 3D X-ray reconstructive technology, scientists analyzed several ostracod specimens, studying their limbs and reproductive organs.
Experts discovered ripe sperm inside the sperm receptacles of a female crustacean, who would have stored the sperm for release once her eggs had matured had she not been encased in the sticky tree resin.
"This female must have mated shortly before being encased in the resin," He Wang, from the Chinese Academy of Sciences in Nanjing, said in a statement.
Scientists said the discovery marks the oldest fossil in which sperm cells have been conclusively identified.