"Sen. Obama is a once-in-a-lifetime candidate who will transform our politics and restore America’s standing in the world," Weld said in a statement.
"We need a president who will lead based on our common values and Sen. Obama demonstrates an ability to unite and inspire. Throughout this campaign I’ve watched his steady leadership through trying times and I’m confident he is the best candidate to move our country forward," he said.
Weld joined other prominent Republicans endorsing Obama over GOP nominee John McCain in the campaign’s final weeks, including former Secretary of State Colin Powell, former Minnesota Gov. Arne Carlson and Scott McClellan, former press secretary to President Bush.
Last year, Weld came to New Hampshire to campaign for another former Massachusetts governor, Republican Mitt Romney, who ended his bid for the presidential nomination in February.
Weld was governor of Massachusetts from 1991 to 1997. Before that, he was U.S. attorney for Massachusetts under President Reagan and was later led the criminal division of the Justice Department.