The contract is for two years and has a maximum value of $5.75 million, a source told ESPN's Josina Anderson.
The 29-year-old sat out the 2011 season after being released by Seattle in August.
Curtis Lofton, Atlanta's starting middle linebacker since he was drafted in 2008, will be an unrestricted free agent Tuesday.
Tatupu is a three-time Pro Bowler and played a major role in taking the Seahawks to their only Super Bowl appearance during his rookie season in 2005.
But he struggled with injuries the past few seasons and has seen a decline in production. He missed most of 2009 after tearing a pectoral muscle.
In 2010, Tatupu played in all 16 games, but his 88 tackles were the fewest of his career in a full season, and he needed surgery on both knees in the offseason.
Tatupu was in his prime in his first few seasons, when he recorded more than 100 tackles and went to the Pro Bowl each year from 2005 to 2007.
Information from ESPN.com's Pat Yasinskas and ESPNChicago.com's Jeff Dickerson was used in this report.