"You know, I voted for him in 2008 and I plan to stick with him in 2012, and I'll be voting for he and Vice President Joe Biden next month," he said on CBS' "This Morning."
Asked whether it was an endorsement, he said, "Yes."
Powell praised the president's handling of the economy and ending of the Iraq War.
"I think we ought to keep on the track we are on," he said.
Powell said he had the "utmost respect" for Mitt Romney, but criticized his tax plan.
He said Romney's foreign policy was a "moving target." "One day he has a certain strong view about staying in Afghanistan, but then on Monday night he agrees with the withdrawal. Same thing in Iraq. On every issue that was discussed on Monday night, Gov. Romney agreed with the president with some nuances. But this is quite a different set of foreign policy views than he had earlier in the campaign."
Powell, a Republican who served in President George W. Bush's first term, backed Obama in 2008. He was frequently mentioned as a potential Republican challenger against Bill Clinton in 1996, but decided against it.