Romney made his announcement to supporters via a phone app.
"Mitt's Choice for VP is Paul Ryan," it said and implored backers to spread the word.
The ticket-mates arranged their first joint appearance later in the morning at a naval museum, first stop of a bus tour through four battleground states in as many days.
In a statement issued Friday night, Romney's campaign said only that the running mate would be present at 9 a.m. EDT at the Nauticus Museum. The USS Wisconsin is berthed there — offering a hint about Ryan's selection.
At 42, Ryan is a generation younger than the 65-year-old Romney.
His conservative credentials are highly regarded by fellow Republican House members, while numerous polls found that Romney's own were suspect among the party's core supporters during the primaries of winter and spring.
A seventh-term congressman, Ryan is chairman of the House Budget Committee, and primary author of conservative tax and spending blueprints that the tea party-infused Republican majority approved over vociferous Democratic opposition in 2011 and again in 2012.
It envisions transforming Medicare into a program in which future seniors would receive government checks that they could use to purchase health insurance. Under the current program, the government directly pays doctors, hospitals and other health care providers.
Ryan and other supporters say the change is needed to prevent the program from financial calamity. Critics argue it would impose ever-increasing costs on seniors.
Other elements of the budget plan would cut projected spending for Medicaid, which provides health care for the poor, as well as food stamps, student loans and other social programs that Obama and Democrats have pledged to defend.
In all, it projected spending cuts of $5.3 trillion over a decade, and cut future projected deficits substantially.
It also envisions a far reaching overhaul of the tax code of the sort Romney has promised.
In turning to Ryan, Romney bypassed other potential running mates without the Wisconsin lawmaker's following among rank-and-file conservatives, including Ohio Sen. Rob Portman and former Minnesota Gov. Tim Pawlenty.
Republican officials said Romney had spoken with both men.
Romney and Ryan appeared unusually comfortable with each other when they campaigned together earlier in the year. The former governor eagerly shared the microphone with the younger man and they shared hamburgers at a fast food restaurant.
In making an endorsement before his state's primary last spring, Ryan said, "I picked who I think is going to be the next president of the United States — I picked Mitt Romney. ... The moment is here. The country can be saved. It is not too late to get America back on the right track. ... It is not too late to save the American idea."
Romney was the subject of an April Fools prank in which Ryan played a role. Romney showed up at a supposed campaign event where he heard Ryan calling him "the next president of the United States" — only to find the room nearly empty.