Williams, who was a restricted free agent, averaged 11.5 points in 23 minutes per game last season.
The 21-year-old Williams was selected out of high school with the No. 45 overall pick in the second round of the 2005 NBA draft. He's averaged 7.2 points, 2.2 assists and 1.5 rebounds in his first three seasons.
Philadelphia signed two-time All-Star forward Elton Brand earlier this summer, and added free-agent guards Royal Ivey and Kareem Rush last week. Forward Andre Iguodala remains a restricted free agent.
