Per ESPN's Dan Graziano, Thomas will be activated to the Seahawks' 53-man roster to start Sunday's season opener.
Thomas sat out all of training camp and the preseason while seeking a new contract from the Seahawks. The three-time All-Pro has one year remaining on his current deal worth $8.5 million.
When no deal materialized, Thomas announced Wednesday on Instagram that he would be returning to the team.
"I worked my whole life for this," he wrote. "... I've never let me teammates, city or fans down as long as I've lived and don't plan on starting this weekend. With that being said, the disrespect has been well noted and will not be forgotten. Father Time may have an undefeated record but best believe I plan on taking him into triple overtime when it comes to my career."
Head coach Pete Carroll told reporters during a conference call it was unclear if Thomas would be ready to play in Week 1.
"It's an adjustment period, as he jumps in with us now," Carroll said, according to 247Sports' Derek Lewis. "We'll figure out how that works, and we'll get through it. He's already on the field with our guys, and he'll practice today. So we'll see what that all means."
Thomas has been an integral part of Seattle's success since he was drafted 14th overall in 2010. He has recorded at least 64 combined tackles and six passes defensed in each of his eight NFL seasons.