Ryan, best known for his proposed budget that calls for an overhaul of Medicare, is considered a rising star in the GOP.
Romney made the official announcement at a campaign stop in Norfolk, Va., calling Ryan "an intellectual leader of the Republican Party."
Pennsylvania Republican leaders praised Romney's choice of running mate.
"I congratulate Paul Ryan on being selected to join Mitt Romney on the Republican Presidential ticket," said Rob Gleason, chairman of the Republican Party of Pennsylvania, in a statement released following the announcement. "Pennsylvanians will be well served by a Romney-Ryan team in a White House that can finally shut the door on President Obama’s four years of high unemployment, out-of-control spending, and usher in a new era of American prosperity.
" Pennsylvania will be first in line to send Mitt Romney and Paul Ryan to the White House when we deliver our 20 electoral votes this November."
“Quite simply, President Obama has failed," continued Gleason. "The unemployment rate has now been above 8 percent for 42 straight months. Friday’s jobs report illustrates how Obama has failed to fix the economy and meet his own standards: He promised his massive $831 billion stimulus would create millions of jobs and bring unemployment down to around 5.5 percent by today.
"Most staggering of all, the real unemployment rate is 15 percent," Gleason added. "That means that 15 percent of Americans either can’t find a job, full-time work or have given up even trying to find a job. That is this President’s legacy and the reason Pennsylvanians are going to vote for the Romney-Ryan ticket."
Pennsylvania Democrats also issued a statement on Romney's choice.
"Mitt Romney's selection of Paul Ryan proves once again that he has the wrong agenda for middle-class Pennsylvanians," said state Democratic Party Chairman Jim Burn in the statement.
"Paul Ryan shares Mitt Romney's commitment to helping the wealthy at the expense of the middle class," said Burn, of Allegheny County and a former president of Allegheny County Council.
"Ryan is the author of a budget plan that will increase taxes for the middle class and end Medicare as we know it, all to fund tax cuts for the wealthiest Americans," he said. "The Romney-Ryan agenda will cut investments in education, clean energy and innovation that will help our economy grow while returning to the same failed trickle-down economic policies of the past. The Romney-Ryan budget plan will cost America over 1 million jobs while cutting key investments in middle class programs like Head Start and veterans health care.
"Mitt Romney's choice of Paul Ryan has made it clear that he will abandon the middle class and seniors and take us back to the same failed policies of the past that hurt middle-class Pennsylvanians."