Willis is entering his 21st season in the league and is second among all active NBA players in games played. The 7-footer was a reserve for San Antonio last season and is expected to play behind center Jason Collier this year.
"I think this is a great opportunity as my career covers two decades in the league," Willis said. "To be back with the franchise I started my career with is a wonderful thing, and I hope to do whatever I can to try and bring more excitement and positive energy to the team."
In 11 years with the Hawks, Willis averaged 14.5 points and 10.0 rebounds. He was the team's first-round pick in 1984 and was an All-Star in the 1991-92 season, when he averaged 18.3 points. He averaged a career-high 19.1 points during the 1993-94 season, his last full year with the Hawks.
"He will be an important presence on the court as well as in the locker room, and we'll look to him especially to set an example for our younger players," general manager Billy Knight said. "His experience and leadership will be invaluable."
Among active players, only Karl Malone has played in more games. Willis spent the past two seasons in San Antonio, averaging 3.4 points and 2.0 rebounds last year in 7.8 minutes a game.
Willis has also played for Miami, Golden State, Houston, Toronto and Denver. For his career, Willis has averaged 12.3 points and 8.5 rebounds.