Williams, who has played his entire seven-year career with Philadelphia, grew up 25 miles northeast of Atlanta in Snellville. He signed to play at Georgia before entering the 2005 NBA draft directly out of South Gwinnett High School.
Financial terms of Williams’ deal were not announced by the Hawks.
Williams, 25, has career averages of 11.3 points, 3.0 assists, and 2.0 rebounds in 455 games.
Atlanta’s busy offseason has included trading Joe Johnson and Marvin Williams and acquiring guards Devin Harris, Anthony Morrow, Jordan Farmar and DeShawn Stevenson and forwards Jordan Williams and Johan Petro.
Lou Williams led the Sixers in scoring last season with a career-best 14.9 average as the sixth man.
The Hawks were waiting for the NBA to approve the signing before they announced the trade.