Washington | Presumptive Democratic presidential candidate Joe Biden has chosen Kamala Harris as his running mate, a proven political battler who becomes the first Black woman in US history on a major party ticket.

Ms Harris, a 55-year-old former rival for the Democratic nomination who clashed with Mr Biden last year as the party's contest began in earnest, was announced by the former vice president on Tuesday (Wednesday AEST), ending an extended selection process that began more than four months ago.

"I've chosen Kamala Harris as my running mate.

Her selection comes after months of social unrest across America over racial inequality and police brutality, which saw Mr Biden come under massive pressure to select a woman of colour as his running mate.

Mr Biden can "unify the American people because he's spent his life fighting for us", Ms Harris wrote in a tweet.

Mr Biden revealed that his decision after weeks of speculation that saw more than a dozen names emerge as potential running mates may have been swayed by Ms Harris's work with his late son Beau, both of whom were state attorney's general.

Regarded as a relative political moderate within the Democratic party, Ms Harris edged out former Barack Obama national security adviser Susan Rice, House of Representatives congresswoman Karen Bass and several other women considered by Mr Biden and his team.