The deal includes $12.32 million guaranteed, according to the source.
As the Seahawks training camp started Saturday, Thomas was waiting at an Olive Garden Italian restaurant. The 21-year-old safety from Texas tweeted he was "waiting patiently ... seems like forever."
The Seahawks top pick, Russell Okung, remains unsigned. Okung was taken with the sixth overall pick in the draft.
"We're checking hourly to see how we're making progress," head coach Pete Carroll said Saturday.
"But our focus goes to the guys who are here, of course. ... I know we made very strong attempts to get this done prior to camp."
The Associated Press contributed to this story.