In a statement released on his Web site, the former world No. 1 thanked Williams and indicated a replacement hasn't been chosen.
"I want to express my deepest gratitude to Stevie for all his help but I think it's time for a change," Woods said. "Stevie is an outstanding caddie and a friend and has been instrumental in many of my accomplishments. I wish him great success in the future."
The 35-year-old is ranked 20th in the world and hasn't won an event since a November 2009 car accident outside his Florida home led to widespread revelations of infidelity to his wife.
He hasn't competed since May at the Players Championship, when he withdrew with a leg injury before completing the first round.
The statement said Woods is continuing to rest and rehabilitate his left leg, which he injured while competing in the Masters, adding, "No decision has been made when he will return to competition on the PGA Tour."