Gabbard, who endorsed Vermont Sen. Bernie Sanders in 2016, never became a major factor for the Democratic Party's nomination. Her departure leaves just Biden and Sanders in the race.
Biden dominated the vote all three states that held primaries on Tuesday -- Florida, Illinois and Arizona.
"After Tuesday's election, it is clear that Democratic Primary voters have chosen Vice President Joe Biden to be the person who will take on President Trump in the general election," Gabbard said in a statement Thursday.
Gabbard finished a distant third in all three states Tuesday, and never won any of the previous primaries. Her best finish was a second-place showing in American Samoa, her birthplace.
"I know Vice President Biden and his wife and am grateful to have called his son Beau a friend who also served in the National Guard. Although I may not agree with the vice president on every issue, I know that he has a good heart and is motivated by his love for our country and the American people," Gabbard said.
Gabbard also acknowledged Sanders, who is well behind Biden in the delegate count so far.
"I have a great appreciation for Sen. Sanders' love for our country and the American people and his sincere desire to improve the lives of all Americans," she said.
Gabbard has already said she won't run for re-election in the House in November.