The two specimens are not much more than an inch long, but they will nevertheless enter the record books as the oldest example of animals with a backbone.
A team of Chinese scientists led by Professor Degan Shu of Northwest University in Xi'an, and aided by Professor Simon Conway Morris of Cambridge University, found the fish in rock sediments in the Chengjiang region of Yunnan Province in southern China.
The site has already proved to be a treasure trove of fossils dating to the Cambrian period between 500 million and 540 million years ago, when life on Earth went through an explosive growth in evolution.
The two specimens have the typical zig-zag muscle formations of fish and possess fins, mouths and possibly eyes - all features of more advanced vertebrates.
Details of the find, published in Nature, show that one of the new fish, called Haikouichthys, has gills supported by gill bars and the second, Myllokunmingia, has more primitive gills decorated with a series of pouch-like structures.
Professor Conway Morris, an authority on the Cambrian period, said that finding animals so old and yet so easily recognisable as fish was "pretty spectacular".
Previous research indicated that fish did not evolve from their more primitive ancestors until at least 50 million years later than the age of the Chengjiang fossils.
"These two specimens are of the greatest importance. Humans are vertebrates, as are rabbits, eagles and frogs, and as such we all evolved from the fish," Professor Conway Morris said. "Until now the early history of the fish has been extremely sporadic and sometimes difficult to interpret. This discovery shows that fish evolved much earlier than was thought."
The Cambrian period is spectacularly rich in fossils, which has led biologists to believe that something special happened to trigger a burst in evolution - leading to the formation of new animal groups such as vertebrates.
Finding fish 50 million years older than was thought possible indicates that the rate of evolution in the oceans during the Cambrian period must have been exceptionally fast, Professor Conway Morris said. "Not only do we see the appearance of the fish, but also a whole range of different animal types. Animals were moving faster and hunting more effectively, and correspondingly other animals were busy developing protective skeletons," he said.