"The time has arrived for Republicans to choose a presidential nominee; a new standard bearer who has the ability and determination to defeat President (Barack) Obama and the strongest commitment to returning America to prosperity and defending our interests and values overseas," McCain said.
"I'm pleased to have made my choice, and to endorse Governor Mitt Romney for the Republican nomination for President," McCain said while endorsing Romney, who on Tuesday won the Iowa Caucus by just eight votes.
Endorsement by McCain, who was the Republican presidential nominee in the 2008 elections, is expected to further boost Romney's presidential campaign.
"Romney offers us the commonsense reforms of government policy that are necessary to turn around our economy," he said.
"His record of accomplishment in government and business are a testament to his leadership abilities. His commitment to a strong defence and principled diplomacy will earn the world's respect for American leadership," he said.
"I have no doubt he is the best candidate we can nominate. And I am proud to support him for President," McCain said.
"Few Americans have sacrificed so much for their country -- it is an honour to have Senator & Mrs McCain's support," Romney said.
"For his entire life, Senator McCain has been a committed public servant and is an example for all those seeking higher office. Senator McCain has been a leader in the fight to stop the growth of government and make government more accountable to taxpayers," he said.
"With his leadership on cutting spending in the Senate, we can rein in out-of-control federal spending, and restore confidence in government. I look forward to campaigning with Senator McCain and working with him on the issues important to our national and economic security," Romney said.
Image: Mitt Romney is joined by John McCain at a campaign stop in Manchester, New Hampshire, on Wednesday
Photograph: Brian Snyder/Reuters