Promoter Dan Goossen said the surgery lasted 6 ½ hours Friday night.
Goossen said the surgery "went as planned" and doctors said they would talk with Williams about the procedure on Saturday.
The 30-year-old Williams severed his spinal cord when he was thrown from his motorcycle Sunday morning in Marietta, Ga. Police said he was driving too fast for conditions.
Williams' manager, George Peterson, said the former welterweight champion has no movement from the waist down.
Known as "The Punisher," Williams is from Aiken, S.C. He had been scheduled to fight Sept. 15 in Las Vegas against Saul "Canelo" Alvarez.
Williams won his first major welterweight title in July 2007 by beating Antonio Margarito.
Goossen said Williams has been talking and in good spirits as he predicted he would walk again.
Williams told Ringtv.com his message to his fans is, "I'm blessed.
"To be conscious and alive and all of that," Williams said. "To know what's going on. I've got a little spine injury, or whatever. But they're working on that. But to be able to be breathing on my own ... I feel real good and I love the support that I've been getting from my fans."