Biden was discussing the empty seat on the Supreme Court at the American Constitution Society for Law and Policy's 2016 National Convention in Washington when he weighed in on the 2016 race.
"Anybody who thinks that whoever the next president is -- and God willing, in my view, it will be Secretary Clinton," Biden said.
A Biden source said there's "no question" this was an endorsement and expects the vice president to say more about Clinton in the coming days.
Clinton thanked Biden for his backing in a tweet Friday afternoon, referencing the vice president's infamous hot-mic moment
in 2010 when he called the passage of Obamacare a "big f---ing deal."
"Thanks, @VP Biden! Your support is a BIG... deal. -H," Clinton tweeted.
The vice president and Warren -- who also endorsed Clinton Thursday -- spoke in front of a friendly and receptive crowd and each hit Trump for his comments on Indiana-born judge Gonzalo Curiel, who is hearing a case concerning Trump University and has been attacked by the GOP candidate due to his Mexican heritage.
Biden called Trump comments about the judge "reprehensible" and "racist."
"I don't think the framers envisioned a presidential candidate accusing a judge of being incapable of reaching a fair decision because of his ethnic (heritage)," Biden said.
Moments earlier, Warren called Trump a "fraud" and "thin-skinned, racist bully."
"Donald Trump is a loud, nasty, thin-skinned fraud who has never risked anything for anyone and serves nobody but himself," the Massachusetts Democrat said.
"Like all federal judges, Judge Curiel is bound by the federal code of judicial ethics not to respond to these attacks," Warren said. "Trump is picking on someone who is ethically bound not to defend himself, exactly what you would expect from a thin-skinned, racist bully."
Trump responded to Warren's broadside on Twitter Friday morning when he mockingly called her "Pocahontas," reviving a 2012 controversy how the Massachusetts senator described her Native American heritage.
"Pocahontas is at it again! Goofy Elizabeth Warren, one of the least productive U.S. Senators, has a nasty mouth. Hope she is V.P. choice," Trump wrote.
President Barack Obama endorsed Clinton earlier on the day of Warren's speech. Obama and Biden met separately Thursday with Sen. Bernie Sanders, Clinton's rival in the Democratic race.
Warren and Biden also criticized Senate Republicans, blaming them for their reluctance to hold hearings for federal judges -- including Supreme Court nominee Merrick Garland -- while at the same time distancing themselves from Trump for his comments about Curiel.
The speeches reflect the unusually prominent role the judiciary is playing this election at a time when an eight-member Supreme Court is rushing to issue opinions that could reverberate on the campaign trail on issues such as affirmative action, abortion and immigration.
For her part, Warren referred to a "full scale assault" on the integrity of the federal judiciary and its judges. "The Supreme Court sits paralyzed," she said, "unable to deal with its most challenging cases" because of "extremist Republicans" she accused of "rejecting the legitimacy of President Obama."
Senate Majority Leader Mitch McConnell on Wednesday defended the actions, saying that Obama has had many more judicial nominees confirmed than President Bush did at the same point in his presidency. "We'll continue to process his judicial nominations, but the minority is not going to dictate to the majority when and how we will do so," he said.
According to the Administrative Office of the U.S. District Courts, there are currently 91 vacancies across the federal judiciary with 60 nominees pending. Of that number 30 are considered "judicial emergencies" defined by the office as vacancies with a high number of filings or where a there is more than one authorized judgeship and only one active judge.
Speaking to an audience of lawyers and judges Warren also brought up Garland, saying, "he is not a politician. He is a judge with an unimpeachable record of putting the law first."
"For that sin," she continued, "he faces a nonstop, national campaign of slime." She called it "despicable."
Warren then lambasted McConnell for failing to immediately criticize Trump, linking the two issues.
"When first asked if he would condemn Trump's comments about Judge Curiel, Sen. Mitch McConnell, the Senate Republican leader, said, well, gee, you know, 'Donald Trump is certainly a different kind of candidate.' After days of pressure, McConnell finally said that attacking the judge is 'stupid' and that Trump should 'get on script.'"