After the ruling by the Cardiff coroner, a reward is likely to be paid to the detectorist and landowner.
It is hoped the coins will be donated to National Museum Wales, which has called the find "sensational".
Two different hoards were found by the metal detectorist on successive days, one involving 2,366 coins and the other 3,547 coins, 3m away.
The 1,700-year-old coins dated from the reigns of many emperors, notably Constantine I (the Great, AD 307-37), during whose time Christianity was first recognised as a state religion.
Derek Eveleigh, 79, from Penarth, who came across the hoards in a field of cows, has kept his find a secret until the outcome of the inquest.
He said: "I had a signal first and when it was deep I thought I should dig it - and that was it."
The coins now will be valued by an independent group
Edward Besly, the museum''s coin expert called it an "exceptional find".
He said: "The coins provide further evidence for local richness at the time. They also shows the complex imperial politics of the early fourth century."
''Time of danger''
It is thought the two hoards were buried by the same person, possibly three years apart. A similar find was unearthed in the area in 1899.
"There was lots of Roman activity in the area at the time, southwards from Cardiff Castle, where there was a Roman fort, to the Knap at Barry where there was an administrative house and there were farms in the Sully area," said Mr Besly.
"There''s a human story there somewhere but it''s intangible, we can''t really get to it but surely somebody buried two pots of coins."
"It might have been they were buried for safe keeping, possibly at a time of danger."
It is assumed the coins will be given over to the museum for further study and to go on public display.
Mr Eveleigh added: "All I found before was thrupenny fragments and bits of metal.
"I have had to keep it quiet all these weeks. Someone told me that when you find one hoard you find more."
Also announced treasure by the coroner were two bronze axes from Llancarfan, Vale of Glamorgan.