After a rare public appearance in Washington, Bush offered his backing in response to a question posed by ABC News as he exited the stage Tuesday.
"I'm for Mitt Romney," Bush responded.
Bush's return to the nation's capital lasted only a few hours to speak about democracy advocates and promote a human rights campaign that's part of the George W. Bush Presidential Center in his home state of Texas.
"I actually found my freedom by leaving Washington," Bush said, adding that it felt good to "be back and to see old friends."
Romney campaign spokeswoman Andrea Saul said, "We welcome the president's support, as we welcomed his father's" backing in late March, when former president George H.W. Bush proffered his formal endorsement, The Washington Post said.
"We're proud to have the president's support," she added, "but he made clear when he left office that he was not going to engage in political campaigns and we have no reason to believe that is going to change."
John Simon, a one-time national security special assistant to Bush, still considered toxic in some Republican quarters, told ABC News he didn't think Bush would be campaigning this fall.