The former president offered a four-word endorsement of Romney as the doors of his elevator were closing after a speech Tuesday in Washington.
Bush said: "I'm for Mitt Romney."
Bush said after the speech, prompting his unscripted -- but not surprising -- endorsement.
Bush's parents, former President George H.W. Bush and Barbara Bush, endorsed Romney in March during an appearance in Texas. Former Florida Gov. Jeb Bush also publicly backed Romney.
Romney's campaign issued a brief statement welcoming the support.
Since leaving office in January 2009, George W. Bush has tried to avoid politics. He left Washington deeply unpopular with voters who blamed him for the economic crisis that unfolded on his watch.