"He has my vote," the Rev. Jackson told The Associated Press in a telephone interview.
Jackson sought the Democratic presidential nomination in 1984 and 1988, winning 13 primaries and caucuses in 1988. His son, Rep. Jesse Jackson Jr. of Illinois, already has endorsed Obama.
Jackson represents a different era of black politician, battle-tested by the civil rights struggles of the 1960s with Martin Luther King Jr.
Obama, 45, is biracial — his white mother was from Kansas, his father Kenyan — and educated at Ivy League universities.
In his best-selling memoir, "Dreams From My Father," Obama said he couldn't even get in the door at national civil rights groups when he was younger. He wrote letters to them after graduating from Columbia University but said none responded.
In a statement responding to Jackson's support, Obama said, "This campaign has been about giving hope since Day One and I am proud to have the support of my friend Jesse Jackson. It is because people like Jesse ran that I have this opportunity to run for president today."
Jackson could help Obama to secure the support of black voters, a critical bloc in the Democratic primaries.
Jackson has a long history with one of Obama's chief rivals, Sen. Hillary Rodham Clinton and her husband former President Clinton. He counseled the two when the president's affair with former White House intern Monica Lewinsky became public.
But Jackson said his history with the Clintons doesn't complicate his decision to back his home state senator, calling Obama Illinois' "favorite son."
"It's not awkward at all," he said, adding, "I don't owe a debt to any of them."
Jackson said he will support the winner of the Democratic nomination, whether it's Obama or not, and he is talking to other candidates because of his agenda that includes the war on poverty and voter protection.
Although Jackson failed in his bids for the White House in 1984 and 1988, he said that helped make it easier for not only blacks, but women and other minorities to run for president and function at the highest levels of government.
"We broke down barriers," Jackson said.
Jackson said Obama has not asked him to campaign for him and he is not in Obama's inner circle of advisers and fundraisers.
"I just have an appreciation of him," Jackson said.