Thomas is set to make $8.5 million entering the final season of a four-year, $40 million deal he signed in 2014. The six-time Pro Bowl selection has skipped the organized team activities of the Seahawks, who begin minicamp on Tuesday.
"I will not be attending the upcoming minicamp or any team activities until my contract situation is resolved," Thomas wrote on Twitter. "I want everyone especially the 12s to know that I want to remain a Seahawk for the rest of my career but I also believe that based on my production over the last 8 years that I've earned the right to have this taken care of as soon as possible. I want to have certainty in regards to the upcoming years of my career.
"I'm going to continue to work my craft and put in work so that I can add to the team and give us the best chance to win. I hope my teammates understand where I'm coming from I believe this is the right thing to do."
Thomas would be subject to up to $84,435 in fines if he were to skip the entire three-day minicamp, though teams can decide against imposing them.
When Thomas signed the contract in 2014, it ranked first among safeties in terms of annual average. Now, it is sixth, according to Spotrac.com.
Seahawks general manager John Schneider said the team's precedent of completing extensions with core players before they conclude their final contract seasons does not apply to Thomas. Schneider said that is due to Thomas being on his second contract.
Thomas, a three-time first team All-Pro, has made 642 tackles, 25 interceptions and five fumble recoveries after being selected by the Seahawks with the 14th overall pick in the 2010 NFL Draft out of Texas.
His holdout will be the Seahawks' first since 2015, when safety Kam Chancellor sat out through training camp and missed the first two games of the season.