The choice, announced by Romney Saturday morning from the deck of the retired USS Wisconsin in Norfolk, Va., appears to show the presumptive Republican nominee will move from attacks on Democratic President Barack Obama to discussion of the nation's fiscal future, The New York Times reported.
The likely GOP ticket is on a four-day, four-state bus tour of Virginia, North Carolina, Florida and Ohio, all swing states in the November general election.
The 42-year-old Ryan is seen as a rising star by Republican conservatives. He was the architect of the House budget deficit reduction plan and is a staunch opponent of the president's healthcare law.
His budget plan, once held at arm's length by many Republicans, is now the foundation of the party's fiscal platform.
Analysts saw the selection of Ryan as Romney bringing to the forefront his ideas for a conservative restructuring of the nation's economic priorities.
Ryan sits on two powerful House committees that hold significant power in shaping the federal budget. He's head of the Budget Committee and a senior member of the Ways and Means Committee.
Ryan's budget plan has already been a target of criticism by Obama, who called it "a vision of our future that's deeply pessimistic."
Romney had vetted others before deciding on Ryan, including Tim Pawlenty, the former governor of Minnesota, and U.S. Sen. Rob Portman of Ohio. His decision was helped by the rapport that was quickly established between the two men. They spent five days campaigning together in Wisconsin in April, and Romney jokingly denied at one point that Ryan was one of his sons.