The senator said in a Sunday statement that she would "do everything in my power" to help Biden win.
"There is no one better prepared than Joe to steer our nation through these turbulent times, and restore truth, honor, and decency to the Oval Office," Harris said in a statement Sunday. "He is kind and endlessly caring, and he truly listens to the American people."
The decision by the California senator who was one of three black candidates seeking to challenge President Donald Trump further solidifies the Democratic establishment's move to close circles around Biden after his Super Tuesday success. Her endorsements comes before the next round of primaries, with six states voting Tuesday, including Michigan.
Harris joins Sen. Amy Klobuchar, D-Minn., former South Bend, Ind. Mayor Pete Buttigieg, former Texas Congressman Beto O'Rourke, ex-New York City Mayor Mike Bloomberg, Rep. Tim Ryan, D-Ohion, former Massachusetts Gov. Deval Patrick, and former Maryland Congressman John Delaney in endorsing Biden.
Harris withdrew from the race in December, ending a candidacy with the historic potential of becoming the first black woman elected president.
Harris said in her statement that "like many women, I watched with sadness as women exited the race one by one." Four years after Hillary Clinton was the party's nominee, "we find ourselves without any woman on a path to be the Democratic nominee for president."
Rep. Tulsi Gabbard, D-Hawaii, remains in the race, but with only a single delegate so far, has been viewed as a long-shot at best.
The Associated Press contributed to this report.