It's considered to be one of the most complete Tyrannosaurus Rex skeletons ever found. It's 85 percent intact, including the skull.
The skull is about 4 feet long with teeth the size of bananas. The T-Rex itself is 40 feet long.
It's called the Wankel T-Rex, named after Kathy Wankel, who discovered it on a Montana Wildlife Refuge in 1988.
The bones were on federal land and are the property of the U.S. Army Corps of Engineers.
The corps loaned the skeleton to the museum of the Rockies in Montana from 1990 to 2011 and now it'll be on loan to the Smithsonian for 50 years.
"Not that many skeletons of this animal - so to have a real one here in Washington at museum is going to be great," says Kirk Johnson, the Sant director.
In October, the bones will be carefully packed up and brought by a shock-absorbed truck to D.C.
They'll arrive on October 16th, which is National Fossil Day.
And next April, sections of the T-Rex will be put on display around the museum, while the dinosaur exhibit undergoes a 5-year, $48 million renovation.
The full skeleton will make its debut when the new exhibit opens in 2019.