Williams, an Atlanta area native, averaged a career-high 14.9 points and 3.5 assists per game with the Philadelphia 76ers this past season, when he finished second in the Sixth Man of the Year voting.
"...happy to be apart of a team that's similar to the Sixers...Young, athletic, and hardworking," Williams wrote on his Twitter feed Tuesday evening.
After talks with the 76ers broke down, Williams announced last week that he wouldn't be returning to Philadelphia. The 76ers, who originally drafted Williams out of high school in the second round of the 2005 draft, later filled Williams' spot by agreeing to a contract with free agent guard Nick Young.
Williams, who can play both guard positions, opted out of the final year of his contract worth $5.3 million. The Hawks are believed to be signing him with their $5 million midlevel exception.
It is part of an offseason overhaul Atlanta has undergone under new general manager Danny Ferry. Since July 1, the Hawks have agreed to trade All-Star Joe Johnson to the Brooklyn Nets for a package of players and a first-round pick, and have a deal in place to trade forward Marvin Williams to the Utah Jazz for guard Devin Harris.
John Jenkins, the Hawks' first-round pick, signed his rookie contract minutes before joining Tuesday's opening of the team's rookie camp.
The sharp-shooting rookie made a good first impression on coach Larry Drew.
"He's a guy we're really looking to come in here and put the ball in the basket," Drew said. "I mean, that's what he does. He's one of the best I've seen in quite some time as far as shooting the basketball."
Jenkins led the Southeastern Conference in scoring two straight years at Vanderbilt.
Information from The Associated Press was used in this report.