The Clippers also are sending two future second-round draft picks and cash considerations in the deal.
Rondo, 35, gives the Clippers championship experience, leadership and basketball IQ at point guard. He returns to Los Angeles, where he won his second NBA title last season with the Lakers before joining the rebuilding Hawks on a two-year, $15 million deal.
Rondo is also reunited with Clippers coach Tyronn Lue, who was an assistant with the Boston Celtics from 2011 to '13, when Rondo was with the Celtics.
Speaking Thursday night, Lue called Rondo "pretty much my project" while in Boston.
"He's fiery, that's what we need," Lue added. "He's tough. He understands the game. And he respects people as a straight shooter and will tell him the truth. That was my biggest thing in Boston, was just being honest with him and telling him the truth and keeping it real with him."
Lue said he is unsure if Rondo will start or come off the bench.
"We've got to get everybody together, get everyone healthy," Lue said after the short-handed Clippers beat the Spurs 98-85 on Thursday night. "Just trying to see what works. It's going to take some time. Just not sure right now.''
Williams has won three of the past six Sixth Man of the Year awards and just passed 15,000 career points Wednesday night in the Clippers' win at San Antonio.
During Thursday's game, Lue said he looked down at his bench during a sluggish start and told his assistants this is when he would want Williams.
"He's going to be missed for everything he brought and not just basketball," Lue said. "I just think he was a great person. That's what I love the most. We all knew he was a great basketball player, but just him as a human being and what he stood for and how he always boosted the morale of the team with everyone around him. That's what we are going to miss the most.''
Clippers forward Paul George called trading away Williams "a tough loss."
"You hate to see these things happen," George said. "We're going to miss Lou. The best way we can do this, to send him off well, is to go out and try to win it [all]."