The 7ft (2.1m) sarcophagus was being used as a trough to stand flowers in in the garden of a house in Dorset, where its significance was recognised by Guy Schwinge, a Dorchester auction valuer.
Mr Schwinge described how he spotted the coffin “peeping out from under some bushes” during a routine valuation.
“As I drew closer I realised I was looking at a Roman sarcophagus of exceptional quality,” he said.
Mr Schwinge, of Duke’s in Dorchester, discovered the family had acquired the sarcophagus almost 100 years ago at auction.
An auction catalogue from 1913 shows the coffin was imported to Britain by Queen Victoria’s surveyor of pictures, Sir John Robinson, who lived at Newton Manor in Swanage, Dorset.
“When I saw the name Duke’s on the front (of the catalogue) I couldn’t believe it,” Mr Schwinge said.
The rectangular sarcophagus is carved from fine quality white marble, said a spokesman for Duke’s, who sold the coffin for a second time.
The quality of the carving suggests it was made for a high status individual.
Experts from the British Museum have estimated the sarcophagus dates from the 2nd Century.
The owners were “utterly delighted” with the sale, Duke’s said.