Three species of creature, which are only a millimetre long and resemble jellyfish encased in shells, were found 2.2 miles (3.5km) underwater on the ocean floor, 124 miles (200km) off the coast of Crete, in an area with almost no oxygen.
The animals, named Loriciferans due to their protective layer, or lorica, were discovered by a team led by Roberto Danovaro from Marche Polytechnic University in Ancona, Italy.
One of the species has been named Spinoloricus Cinzia, after Dr Danovaro's wife, while the other two, known as Rugiloricus and Pliciloricus, have yet to be formally named.
They were found during three expeditions to find life in the sediment of L'Atlante basin in the Mediterranean, which took place over the course of a decade.
Professor Danovaro told BBC News bodies of multicellular animals had been found in sediment from a similarly oxygen-starved area of the Black Sea, but they were thought to have been carried there from adjacent oxygenated water.