“I need someone working alongside me who is smart, tough, and ready to lead,” Biden wrote.
“Kamala is that person. I need someone who understands the pain that so many people in our nation are suffering. Whether they’ve lost their job, their business, a loved one to this virus.”
In a separate tweet he described Harris, 55, as a “fearless fighter for the little guy, and one of the country’s finest public servants”.
Before joining the US Senate in 2017, Harris served as attorney-general of California, the country’s most populous state.
Biden said he had gotten to know Harris well during her time working alongside his late son Beau, who served as Delaware’s attorney-general.
“I watched as they took on the big banks, lifted up working people, and protected women and kids from abuse,” Biden said. “I was proud then, and I’m proud now to have her as my partner in this campaign.”
Biden’s choice was instantly welcomed by prominent Democrats, including former president Barack Obama.
“Joe nailed this decision,” Obama said in a statement, describing Harris as an “ideal partner to help him him tackle the very real challenges America faces”.
“This is a good day for our country,” Obama said. “Now let’s go win this thing.”
Former vice president and presidential nominee Al Gore who described Harris as “an extraordinary running mate”.
“Kamala has made climate and environmental justice a top priority in her career in public service, and I know she will continue to be a strong advocate in the White House,” Gore said in a tweet.
Harris ran for the Democratic Party’s nomination but, despite generating early buzz and putting in some impressive debate performances, she failed to catch fire with voters and dropped out before the Iowa caucuses.
Harris and Biden clashed during the first Democratic debate last year, when she attacked him over his opposition to the mandatory “busing” of black children into white school districts in the 1970s and 1980s.
Despite being personally upset by the attack at the time, Biden later said that he did not hold a grudge against Harris.
Biden announced at a Democratic debate in March that he would select a woman as his running mate, the first time a candidate has made such an explicit pledge.
The vice-presidential announcement is one of the marquee moments each election year, but has generated especially intense interest this year.
Given Biden will be 78 years old by inauguration day, there is a high likelihood he will only serve one term as president – putting his running mate in a strong position to succeed him in 2024.
Describing why this year’s vice-presidential slot is a particularly appealing role, former Democratic senator Claire McCaskill recently said, “You’re writing your ticket to be the first woman president.”
Biden and Harris will officially accept the party’s nomination at the Democratic National Convention next week.
Among the other potential running mates Biden considered were Obama administration official Susan Rice, Michigan Governor Gretchen Whitmer, Illinois Senator Tammy Duckworth and California congresswoman Karen Bass.
Biden rang these women individually earlier in the day to inform them that he had not chosen them as his vice presidential selection.
Biden and Harris will make their first joint appearance on Wednesday (Thursday AEST) at his home state of Delaware.
Matthew Knott is North America correspondent for The Sydney Morning Herald and The Age.