"We must come together to defeat the most dangerous president in modern history," Sanders said on Twitter shortly before making a surprise appearance on Biden's livecast where he endorsed the Democratic former vice president.
Adding that they wanted to make Trump a one-term president, Sanders said from Vermont, "We need you in the White House and I will do all that I can to make that happen."
Biden responded from Delaware, "It's a big deal. Your endorsement means a great deal, a great deal to me."
Although Sanders had ended his campaign last week, he had not endorsed Biden till Monday and had said that he would keep the delegates backing his candidature who had been elected in the party elections, news agency IANS reported.
He had said that he was ending his campaign because "I cannot in good conscience continue to mount a campaign that cannot win and which would interfere with the important work required of all of us in this difficult hour."
Reacting to Sanders's announcement last week, Trump said that many of the senator's supporters agreed with him on bringing back industries to the US and on strengthening its industrial base.
Therefore, he claimed that he would draw their supporters again.
Biden told Sanders "I'm going to need you, not just to win the campaign, but to govern."
Sanders asked him to adopt some policies that are popular with his young supporters like free community college, easing of education loans and a national minimum wage of $15.
They agreed that their staff would work on the agenda.
Both of them criticised Trump's leadership in dealing with the coronavirus crisis.
Sanders called Trump a racist, a xenphobe, an anti-women person and a religious bigot who should not be allowed another term.
The US presidential election is scheduled to take place on November 3, 2020.
There were 20 candidates when the campaign for the Democratic Party nomination began last year but all have dropped out. Sanders was the last holdout.