Obama announced the pick on his Web site with a photo of the two men and an appeal for donations.
A text message went out shortly afterward that said, "Barack has chosen Senator Joe Biden to be our VP nominee."
Biden, 65, has twice sought the White House, and is a Catholic with blue-collar roots, a generally liberal voting record and a reputation as a long-winded orator.
Across more than 30 years in the Senate, he has served at various times not only as chairman of the Foreign Relations Committee but also as head of the Judiciary Committee, with its jurisdiction over anti-crime legislation, Supreme Court nominees and Constitutional issues.
In selecting Biden, Obama passed over several other potential running mates, none more prominent than former first lady Hillary Rodham Clinton, his tenacious rival in dozens of primaries and caucuses. Clinton issued a statement Saturday praising Obama's decision and calling Biden "an exceptionally strong, experienced leader and devoted public servant."
Obama's campaign arranged a debut for the newly minted ticket on Saturday outside the Old State Capitol in Springfield, Ill.
Obama's decision leaked to the media several hours before his aides planned to send a text message announcing the running mate, negating a promise that people who turned over their phone numbers would be the first to know who Obama had chosen. The campaign scrambled to send the text message after the leak, sending phones buzzing at the inconvenient time of just after 3 a.m. on the East Coast.
Hundreds of miles to the west, carpenters, electricians, sound stage gurus and others transformed the Pepsi Center in Denver into a made-for-television convention venue.
Tucked away in one corner were thousands of lightweight rolled cardboard tubes, ready-made
handles for signs bearing the names of the Democratic ticket - once the identity of Obama's running mate was known.
While Obama decided against adding Clinton to his ticket, he has gone to great lengths to gain the confidence of her primary voters, agreeing to allow her name to be placed in nomination at the convention and permitting a roll call vote that threatens to expose lingering divisions within the party.
Biden slowly emerged as Obama's choice across a long day and night of political suspense as other contenders gradually fell away.
First Virginia Gov. Tim Kaine let it be known that he had been ruled out. Then came word that Sen. Evan Bayh of Indiana had also been passed over.
Several aides to Clinton said the Obama campaign had never requested financial or other records from her.
Other finalists in the veep sweepstakes were Kansas Gov. Kathleen Sebelius and Texas Rep. Chet Edwards.
Among those on the short list, Biden brought the most experience in defense or foreign policy - areas in which Obama fares relatively poorly in the polls compared with Republican Sen. John McCain.
While the war in Iraq has been supplanted as the campaign's top issues by the economy in recent months, the recent Russian invasion of Georgia has returned foreign policy to the forefront.
In addition to foreign policy experience, Biden, a native of Scranton, Pa., has working-class roots that could benefit Obama, who lost the blue-collar vote to Clinton during their competition for the presidential nomination.
Biden was elected to the Senate at the age of 29 in 1972, but personal tragedy struck before he could take office. His wife and their 13-month-old daughter, Naomi, were killed when a tractor-trailer broad-sided her station wagon.
Biden took his oath of office for his first term at the hospital bedside of one of his sons.
On Friday, he spent the day at his home in Delaware with friends and family. The normally loquacious lawmaker maintained a low profile as associates said they believed - but did not know - he would be tapped. They added they had been asked to stand by in case their help was needed.
No sooner had word spread of his selection than McCain's campaign unleashed its first attack.
Spokesman Ben Porritt said in a statement that Biden had "denounced Barack Obama's poor foreign policy judgment and has strongly argued in his own words what Americans are quickly realizing - that Barack Obama is not ready to be president."
As evidence, Republicans cited an ABC interview from August 2007, in which Biden said he would stand by an earlier statement that Obama was not ready to serve as president. Hours later, the McCain campaign released a 30-second TV ad featuring Biden's comments from the interview and scheduled the spot for key states.
Biden is seeking a new Senate term in the fall. There was no immediate word on whether he intended to change plans as he reaches for national office.
Michael Silberman, a partner at online communications firm EchoDitto, said the campaign gambled when they made such a high-stakes promise and find themselves in a precarious situation where they could risk a great deal of trust with supporters.
"For Obama supporters, this is like finding out from your neighbor instead of your sister that she's engaged - not how you want or expect the news to be delivered," Silberman said.
Biden dropped out of the 2008 race for the Democratic presidential nomination after a poor finish in the Iowa caucuses, but not before he talked dismissively of joining someone else's ticket.
"I am not running for vice president," he said in a Fox interview. "I would not accept it if anyone offered it to me. The fact of the matter is I'd rather stay as chairman of the Foreign Relations Committee than be vice president."
He had stumbled on his first day in the race, apologizing for having described Obama as "clean."
Months later, Obama spoke up on Biden's defense, praising him during a campaign debate for having worked for racial equality.
It was Biden's second try for the White House. The first ended badly in 1988 when he was caught lifting lines from a speech by British Labor Party leader Neil Kinnock.
In the decades since, he become a power in the Senate, presiding over confirmation proceedings for Supreme Court nominees as well as convening hearings to criticize President Bush's handling of the Iraq War.
Biden voted to authorize the war, but long ago became one of the Senate's surest critics of the conflict. Ironically, perhaps, his son, Beau, attorney general of Delaware, is due to spend a tour of duty in Iraq beginning this fall with his National Guard unit.
Obama worked to keep his choice secret, although he addressed the issue broadly during the day in an interview.
"Obviously, the most important question is: Is this person ready to be president?" Obama told "The Early Show" on CBS. Second, he said, was: "Can this person help me govern? Are they going to be an effective partner in creating the kind of economic opportunity here at home and guiding us through some dangerous waters internationally?"
And, he added: "I want somebody who is going to be able to challenge my thinking and not simply be a yes person when it comes to policymaking.
--- Associated Press writers David Espo in Denver, Angela K. Brown in Waco, Texas, Glen Johnson in Boston, Randall Chase in Greenville, Del., Bob Lewis in Richmond, Va., John Hanna in Topeka, Kan., Scott Lindlaw in San Francisco and Jesse Holland in Washington contributed to this report. Pickler reported from Chicago. (Copyright 2008 by The Associated Press. All Rights Reserved.) AP-NY-08-23-08 0837EDT