His 2020 bid is expected to be very different to his quest for the Democratic nomination in 2016.
Mr Sanders — who grew up in a Jewish family — was a challenger to former Secretary of State Hillary Clinton, the eventual winner, and won 23 primaries and caucuses.
He said he is running to oppose President Donald Trump and to enact many of the ideas he ran on in 2016, including universal health care coverage, a $15 minimum wage and reducing student debt.
He said: "I think the current occupant of the White House is an embarrassment to our country. I think he is a pathological liar... I also think he is a racist, a sexist, a homophobe, a xenophobe, somebody who is gaining cheap political points by trying to pick on minorities, often undocumented immigrants."
Mr Sanders told the New Yorker in a 2015 interview that growing up in a less-than-wealthy Jewish family had instilled in him the idea that politics mattered.
He said he wanted his latest bid to inspire a "grassroots movement of people prepared to stand up and fight."
