The inscribed lead bar is the only one known of its type to be found the UK, auctioneers have said.
Found by Plymouth bricklayer Jason Baker, 31, it failed to fetch its guide price of £60,000 in November.
But, Hanson's Auction House in Derby confirmed it had sold the 2ft (60cm) ingot, which dates from AD 164 and was uncovered near Wells.
More on the Roman lead bar and other stories from Devon
The ingot, known as a "pig", is thought to have been mislaid by the Romans who at the time would have been sending mined lead back to Rome.
Mr Baker, who had been detecting for 18 months when he made the discovery, was unavailable for comment.
Previously, he said: "Normally, I find just a couple of Roman coins and that's normally a good day."
Mr Baker had signed up for a weekly event with the Southern Detectorists' Club when he made the find.
He said at the time there had been a "frenzy of finds" so when his detector sounded he "knew it was something good".
He also said he would be sharing the money from the sale with the farmer.
A rare ingot of Roman lead that was unearthed on a farm has sold for £25,000.