He took the oath of office in a private event at the White House earlier today. He is due to attend a public swearing-in ceremony at the Pentagon later today.
That event will be attended by U.S. President George W. Bush, who has said he's looking for a "fresh perspective" on Iraq.
Gates, a former CIA director, replaces Donald Rumsfeld, who had served as Bush's defense chief since early 2001.
(compiled from agency reports)