Vice President Joe Biden presided over a private, ceremonial swearing-in on Friday at the State Department. Baucus has also been officially sworn in by Secretary of State John Kerry.
Baucus is a Montana Democrat and former chairman of the Senate Finance Committee. The Senate confirmed his nomination earlier this month.
Baucus has said he wants to help build a better economic relationship with China. The U.S. trade deficit with China is far bigger than it is with any other country, and China is the largest foreign holder of U.S. debt.
The State Department said outgoing Ambassador Gary Locke will remain in his role until Baucus departs the U.S. for China. That date hasnt been announced.