He made his announcement in a video posted on Twitter.
"Everything that has made America America is at stake. That's why today I'm announcing my candidacy for president of the United States," the 76-year-old Biden said in the 3½-minute video.
Biden's first major campaign event is set for Monday in Pittsburgh, where he is expected to speak about rebuilding an inclusive middle class, according to NBC News. Biden will then launch a multistate campaign tour with a focus on states with early voting, where he will hash out his vision for closing the divides that are driving Americans apart, NBC reported.
Thursday's announcement marks the unofficial end of the chaotic early phase of the 2020 presidential season. The field now features at least 20 Democrats jockeying for the chance to take on President Donald Trump next year. Several lesser-known candidates may still join the race.
Biden's decision to run came despite accusations in March and April by several women that he touched them in ways that made them feel uncomfortable. In early April, Biden promised in a two-minute video to be "more mindful and respectful of people's personal space."
Despite entering the race later than most of the field, Biden has consistently led polls of Democratic primary voters ahead of Sens. Bernie Sanders and Kamala Harris, and South Bend, Indiana, Mayor Pete Buttigieg. This is Biden's third run for the White House.
Biden's announcement ends more than two years of speculation about whether he would launch a White House bid for the 2020 election, dating to President Donald Trump's victory over former Secretary of State Hillary Clinton in November 2016.
Biden's decision not to run for president in the 2016 contest helped clear the way for Clinton to become the Democratic nominee.
Some have speculated that that decision, which Biden said was based in part on his enduring grief following the death of his 46-year-old son, Beau, to brain cancer in March 2015, might have cost him his best shot at the presidency.
In fact, if he had entered the 2016 fray, Biden would have started off as the most popular candidate, according to a September 2015 NBC News/Wall Street Journal poll.
With a resume boasting 36 years in the Senate representing Delaware and eight more in President Barack Obama's White House, Biden's status as an experienced Washington insider serves as just one of many contrasts with Trump.
Even before Biden announced his 2020 run, he led early polls of Democratic presidential candidates. His national name recognition and association with Obama, who enjoys high popularity as a White House alumnus, made him a top contender from the earliest stages of the campaign.
—The Associated Press contributed to this report.