McClellan is the second former administration official to back the Democratic candidate in just over a week, following former Secretary of State Colin Powell.
While Powell chose "Meet the Press" to make his news, McClellan did it at the taping of comedian D.L. Hughley’s new talk show, which is premiering at 10pm on CNN on Saturday.
Interviewing McClellan, Hughley said: "We are a new show and your endorsement would probably mean a lot. But don’t look at the fact that I am black or, no pressure. Endorse somebody, damn it!"
McClellan, who was Bush’s chief spokesman from July 2003 to April 2006, said he had always planned to support the candidate that has the "best chance for changing the way Washington works" and getting things done.
"I will be voting for Barack Obama," he said.
McClellan is also giving an extended interview on "Larry King Live" on Friday October 24 to talk about his endorsement.
McClellan grew disillusioned after leaving the administration and wrote a book, "What Happened: Inside the Bush White House and Washington’s Culture of Deception," that surprised many for its blunt talk.
He admitted to being part of a White House effort to shade the truth about the case for war in Iraq.
The White House said Bush was surprised and saddened about the book.