The black granite slab is inscribed with an edict of Pharoh Nektanebos the First (378-362 BC) imposing a 10% levy on Greek gods in favour of a temple to the goddess Neith.
A joint Franco-Egyptian team of underwater archaeologists displayed some of their most spectacular finds at a press conference in Alexandria.
Until its rediscovery last year, it was known only through Greek legends and a handful of ancient history books.
Researchers believe the city was sent to the bottom of the Mediterranean after an earthquake rocked the region more than 1,000 years ago.
Stone tablet
"History is materialising in our hands," Egypt's Culture Minister Farouk Hosni said.
The archaeologists found a city almost untouched by time, full of colossal statues, hieroglyphic tablets and an extraordinary store of gold coins and jewellery.
Among the most remarkable discoveries is a giant stone tablet that pinpointed the lost city's location.
The black granite slab is inscribed with an edict of Pharoh Nektanebos the First (378-362 BC) imposing a 10% levy on Greek gods in favour of a temple to the goddess Neith.
Archaeologists say they have only just begun to probe the extraordinary treasures.
As many as 20,000 pieces are reported to be still on the sea floor.
Rich with culture
"Here was an important pharaonic harbour city and entrance to ancient Egypt," said French archaeologist Franck Goddio, leader of the international mission excavating the sprawling underwater site.
"We have learned so much in just one year."
Team diver Eric Smith described the site as "rich and amazing".
The BBC's Middle East correspondent, Frank Gardner, says Herakleion was a place of worship as shown by the temple of Heracles found beneath the waves and, according to legend, was a place of love.
This is where the beautiful Helena, of Homeric legend, is said to have fled with her lover to escape her jealous husband Menelaos.
The city was rediscovered last year, after a two-year search, in waters six to nine metres (20 to 30 feet) deep.
Archaeologists are now hoping to piece together from the ruins how people lived in this ancient and almost forgotten city.