The 3rd or 4th century panel with a central image depicting a leopard attacking an antelope was unearthed by a team of archaeologists at Dewlish House, Dorset, in 1974.
The site had been known since 1740 when a storm uprooted a tree trunk to reveal tesserae from a villa floor.
The 7ft 9in x 6ft 3in (2.37 x 1.9m) fragment was offered at Duke’s auction in Dorchester on September, 2018, consigned ‘from an important local collection’.
Institutional interest emerged but it finally sold to a dealer for £30,000 against an estimate of £10,000-20,000. After the panel was then sold to an international collector, Dorset County Museum, also in Dorchester, launched a £150,000 appeal to save and display it.
The panel had been subject to an export bar that was due to expire on October 16 last year, but an extension was granted to this January after the museum made its intentions known to buy the mosaic.
The purchase was completed after nearly 100 donations from supporters, including £50,000 from the Arts Council England/V&A Purchase Grant Fund, £40,000 from Art Fund, £30,000 from the Headley Trust, and £10,000 from the Association for Roman Archaeology.
The panel will go on show to the public in new galleries at the museum as part of a £16.1m redevelopment opening later this year.
Dr Jon Murden, Dorset County Museum director, said: “I’m delighted that, once cleaned and conserved, it will take pride of place within the restored historic stairwell at the museum – alongside other internationally significant Roman mosaics that are already in our collection in Dorchester.”