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.
Ryan's selection - as well as Romney's own nomination - will be ratified by delegates to the Republican National Convention that begins on Aug. 27 in Tampa, Fla.
President Barack Obama and Vice President Joseph Biden will be nominated for a second term at the Democratic convention the following week.
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.
In recent days, conservative pundits have been urging Romney to choose Ryan in large part because of his authorship of a House-backed budget plan that seeks to curb overall spending on benefit programs such as Medicare, Medicaid and food stamps.
Republican National Committee finance chairman Ron Weiser of Michigan, said Friday night that Ryan's selection would help Romney win Wisconsin and its 10 electoral votes in the fall. The state typically supports Democrats in presidential contests, and Obama won it handily four years ago.
Ryan has worked in Washington for much of his adult life, a contrast to Romney, who frequently emphasizes his experience in business.
The congressman worked as an aide in Congress, and also was a speechwriter for Jack Kemp, who years earlier had been one of the driving forces behind across-the-board tax cuts that were at the heart of Ronald Reagan's winning presidential campaign in 1980.
Ryan is also well-known for his fiendish physical fitness workouts.
His congressional district in southeast Wisconsin has something of a bipartisan voting record. Obama took 54 percent of the vote there in 2008, while the congressman received 64 percent in winning re-election.
Outside Ryan's home in Janesville, Wis., on Friday night, there was nothing to suggest that the residence belonged to a vice presidential candidate. An Associated Press reporter who knocked just before midnight got no answer. There was a light on in a first-floor room of the two-story brick home atop a hill.
Earlier this week, a Ryan adviser said the congressman, his wife and their three children were preparing for a weeklong Colorado vacation.
Most of Romney's staff learned of the planned announcement during a 10 p.m. EDT conference call Friday about an hour before the campaign issued a statement. The identity of Romney's pick was not disclosed during the call. The campaign had promised that first news of the selection would be delivered via a phone app.
Earlier in the day, Romney's campaign briefed reporters on the bus tour without mention of the impending vice presidential announcement.
The tour will take Romney through North Carolina, Virginia, Florida and Ohio. All are battlegrounds where Obama won in 2008. They hold 75 electoral votes combined, of the 270 needed to win the election.
AP reporters Steve Peoples and Andrew Taylor in Washington and Todd Richmond in Janesville, Wis., contributed to this report.