"This is a moment where we need a leader, a president, like Joe Biden," Clinton said during a virtual town hall.
"I want to add my voice to the many who have endorsed you to be our president," the former secretary of state said, describing Biden as a "friend" and a figure who has been "preparing for this moment his entire life."
Biden called Clinton a "friend" and thanked her for the "wonderful personal endorsement."
One day earlier, House Speaker Nancy Pelosi also endorsed Biden, calling him a "voice of reason and resilience" amid the coronavirus pandemic.
Biden already has the formal support of former President Barack Obama, progressive leaders Bernie Sanders and Elizabeth Warren, and the party's two previous presidential nominees John Kerry and Al Gore.