Democrat Hillary Clinton has become the first woman in US history to be nominated for president by any major party.
She reached the milestone in a roll call vote from all 50 states at the Democratic National Convention (DNC).
Mrs Clinton's nomination comes after Senator Bernie Sanders' supporters disrupted the first day of the convention with boos and jeers.
Party officials have worked to ease tensions amid protest on the floor.
Mrs Clinton passed the 2,382 delegates needed to claim the nomination after South Dakota announced its delegate vote count.
In a symbolic gesture of party unity, former Democratic rival Senator Bernie Sanders took the microphone to move to suspend the procedural rules and declare Mrs Clinton as the nominee by acclamation.
Chair of the Democratic National Committee, Congresswoman Marcia Fudge, approved the motion after a unanimous voice vote and members of the crowd erupted into cheers as they held up new Clinton signs.
Former President Bill Clinton is due to speak about his wife later on Tuesday, making the case for why she is qualified to take his old job.
The second night will focus on race and justice, topics that dominated last week's Republican National Convention in Cleveland.
Sybrina Fulton, the mother of Trayvon Martin, whose death inspired the Black Lives Matter movement, will also take the stage to speak on behalf of Mrs Clinton.
Dissention on the convention floor plagued the first day when Sanders supporters booed throughout the event.
Senator Sanders scrambled to quell the protest by sending a text message and an email to delegates urging them to remain calm.
Mr Sanders later took centre stage as the final speaker on the first night and directly told his supporters that ""Hillary Clinton must become the next president of the United States."
In declaring her the nominee, Mr Sanders echoed Mrs Clinton in a role she played eight years ago after a hard-fought primary.
At the 2008 Democratic National Convention, Mrs Clinton called for a vote for Barack Obama by acclamation, ending the roll call vote in an effort to unite the party behind his candidacy.