The near-complete skeleton of the Tyrannosaurus bataar was handed over to Mongolian officials at a ceremony in New York on Monday.
A Florida fossils dealer in December admitted smuggling the bones, which sold at auction for more than $1m (Â£643,000).
US officials seized the bones last year after Mongolia said they were stolen.
''We are very pleased to have played a pivotal role in returning Mongolia's million-dollar baby,'' US Attorney Preet Bharara said.
''Of course, that million-dollar price tag, as high as it is, doesn't begin to describe the true value of an ancient artefact that is part of the fabric of a country's natural history and cultural heritage.''
Mongolian Minister of Culture, Sport and Tourism Oyungerel Tsedevdamba said they would be setting up a new dinosaur museum, as "we never had dinosaurs' museum before".
The bones, discovered in the Gobi Desert, were smuggled into the US from the UK in March 2010.
Mongolia has laws that protect fossils - they are the property of the state and their export is banned.
The US has returned a 70-million-year-old dinosaur skeleton which was smuggled from Mongolia.