He spent last season as a consultant for the Sonics after serving as the team's general manager from 2001-07.
The 56-year-old Sund will replace Billy Knight, who announced on May 7 he is resigning after six years with the team, the last five as general manager.
Sund inherits a team that ended the NBA's longest playoff drought this year by making its first postseason appearance since 1999. The Hawks lost to Boston in seven games in the first round.
The Hawks do not have a pick in the June 26 draft and have two prominent restricted free agents, shot-blocking forward Josh Smith and sixth man Josh Childress.
Another priority for Sund will be to address the status of coach Mike Woodson, whose contract expires on June 30.
Sund, working as co-chairman of the NBA predraft camp in Orlando, is expected to have his first day in Atlanta next Wednesday.
Most of the Hawks' coaching staff and management team joined Sund in Orlando for a meeting on Wednesday night.
Sund said Wednesday night he will not make any decisions regarding Woodson or the possibility of obtaining a draft pick before having a week to study the team.
"As to where we go from here, most of the basketball staff is here," Sund said. "I met with them tonight in an introductory fashion. I told them my first step here is to be a very good listener for the next week. I need to spend some time listening to ownership, listening to the management staff and then some of the players."
Woodson did not immediately return a phone message.