Brennan, a 25-year CIA veteran, currently serves as Obama's top counterterrorism adviser. The White House says the president will announce Brennan's nomination during an event on Monday afternoon.
At the same event, an administration official says, the president will also formally announce that he is nominating Chuck Hagel as his next defense secretary. Both men must be confirmed by the Senate. (Chuck Hagel to be next defense secretary)
Obama considered Brennan for the top CIA job in 2008. But Brennan withdrew his name amid questions about his connection to enhanced interrogation techniques while serving in the spy agency during the George W. Bush administration.
Brennan denied involvement in the controversial interrogation tactics, including waterboarding, and has spoken out against them.