His former rival, Senator Hillary Clinton, called for his nomination by acclamation from the convention floor during the roll call vote of the states Wednesday.
Some states had already cast their votes for Obama and Clinton. Both names were placed in nomination earlier today, the third day of the convention in Denver, Colorado.
Delegates in the hall cheered and clapped and chanted the Obama campaign slogan "Yes We Can."
Obama arrived in Denver shortly before the roll call vote got underway. His vice presidential running mate, Delaware Senator Joseph Biden, also will be formally nominated today and will accept the nomination when he addresses the forum later Wednesday.
At a meeting ahead of the nomination vote, Clinton released her convention delegates to vote for Obama. Many in the audience yelled "No!" in response. The New York senator said she had already cast her vote for Obama, but said she would not tell her delegates what to do and encouraged them to follow their hearts. A small group of Clinton supporters rallied in Denver today outside the convention site.
Obama will address the convention when he formally accepts the nomination Thursday.
Former U.S. President Bill Clinton will take the stage Wednesday before Biden, a day after his wife issued a ringing endorsement for Democratic presidential candidate Barack Obama.
In a speech Tuesday, Senator Clinton described herself as "a proud supporter of Barack Obama" and urged her party to unite behind his candidacy. But she cautioned that "we are on the same team, and none of us can sit on the sidelines."
Obama watched Clinton's speech at the home of supporters in the western city of Billings, in the state of Montana. He told reporters the speech was "outstanding." A spokesman said he later called the Clintons to express his gratitude.
Republicans hold their national convention next week in the city of Minneapolis-St. Paul, Minnesota, where Senator John McCain will become that party's presidential nominee.
By VOA News