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 hasn’t been announced.
MORE HBJ HEADLINES
Watchdog: Too few air traffic controllers where needed most A $32B tally, but Boeing's 787 costs don't bother Wall Street Czech airline to buy 16 Boeing 737 Max jets Lockheed Martin separating unit, combining it with Leidos Apple forecasts rare sales drop Obama administration loosens Cuba embargo with new measures