Ryan, 45, is the 54th Speaker of the US House of Representatives and replaces John Boehner. He was formally elected by members of the House of Representatives today.
He is second in line to the US presidency after the vice president, should the commander-in-chief be impeached or vacate the Oval Office.
US President Barack Obama called Ryan a day earlier, a presidential spokesman said. "The President is hopeful that he'll be able to work with Congressman Ryan and make progress on behalf of the American people," White House Press Secretary Josh Earnest told reporters at his daily news conference.
Ryan soon after taking over urged the White House to have a fresh start, to move forward. "We are not going to have a House that looks like it's looked the last two years. We are going to move forward. We are going to unify. Our party has lost its vision, and we are going to replace it with a vision," Ryan told reporters.
He was first elected to the US House of Representatives in 1999. He has been in the Congress for 17 years now, during which he has held several top positions in the Congress.
Ryan was the running mate of the Republican presidential candidate Mitt Romney in 2012. In 2008, he voted in favour of the India-US Civil Nuclear deal.
He visited India in 2006 as part of a Congressional delegation led by the then Speaker of House of Representative. He has termed the trip an "eye opener" and called India a rising democratic power.
"I think every member of Congress should go to India to see for themselves the kind of global challenges and opportunities that face us as India emerges. It was a real eye-opener," Ryan said after he returned from India in April 2006.
Ryan was among the first US lawmakers to express condolences after the shooting at a Wisconsin Gurdwara in 2012.