"I move senator Barack Obama of Illinois be selected by the convention by acclamation as the nominee of the Democratic Party," she said, a request accepted by the convention's presiding official, Nancy Pelosi, the speaker of House of Representatives.
Pelosi then announced that Obama had accepted the nomination.
Biden to speak
Delegates at the convention in the state of Colorado are also set to hear speeches on Wednesday from Joe Biden, the vice-presidential nominee and Bill Clinton, the former president.
He has yet to offer a comprehensive endorsement of the new party's presidential candidate.
Al Jazeera's Rosiland Jordan in Denver said tensions remained between Bill Clinton and Obama, with Clinton unhappy at Obama's perceived lack of respect for his record as president.
Biden is expected to take the lead in attacking John McCain, the Republican candidate, who has moved slightly ahead in the polls following weeks of Republican pounding of Obama.
On Wednesday McCain released a new advertisement attacking Obama's foreign policies.