A 70 million-year-old Tyrannosaurus Bataar, unearthed in the Gobi Desert, is to be seized by the US Department of Homeland Security.
A New York court has issued a warrant forcing an auction house to return the fossil to modern-day Mongolia, where it was discovered about seven years ago.
The dinosaur was sold at auction in New York last month by Heritage Auctions, for more than $1m (£600,000).
But the court says it was illegally imported to the US via the UK in 2010.
The skeleton initially found its way to Dorset, in southern England, where a collector kept the bones in a warehouse.
It was then transported to the US under the erroneous claim that the so-called "T. Bataar" had been dug-up in Britain and was worth $15,000.
According to the lawsuit, the prehistoric bones arrived in Gainesville Florida in March 2010.