The endorsement comes after Sanders and Obama did the same.
The Massachusetts senator finished third place in her own state's primary contest.
The billionaire ex-mayor spent over US$400 million on his campaign.
Mike Bloomberg and Elizabeth Warren have failed to breakthrough.
The socialist senator is in a strong spot after winning New Hampshire.
The Iowa Democratic Party pointed to "inconsistencies" in the results.
In an email, Harris said her campaign doesn't have the financial resources to continue.
She spent several years in Montreal as a teen.
Republicans improve their majority in the Senate.
It's a referendum on the first half of the Trump presidency.