His agent asked the Dolphins. Thomas got his answer within minutes.
So on Thursday, Thomas signed a one-day contract, officially worth $1 — but in actuality, worth so much more to Thomas, who long lamented not getting the chance to say a proper farewell when the Dolphins released him two years ago.
With that, he finally said good-bye.
"I'm a little bit nervous right now," Thomas said at the team's training facility in Davie, Fla. "More nervous than any game I ever played."
Only a fifth-round draft pick out of Texas Tech in 1996, Thomas became a star during 12 seasons with the Dolphins. He was one of the faces of the franchise along with the likes of Dan Marino and Jason Taylor, who became Thomas' brother-in-law.
Thomas had 1,038 tackles, 19½ sacks and 17 interceptions in 168 games for the Dolphins, and he was chosen for seven Pro Bowls.
"As the team's owner, I want to welcome him back to the Dolphins and I certainly hope he continues to make his presence felt," Stephen Ross said. "He's a great role model for our players and we're proud of everything he has done throughout his career to represent this franchise. He gave his heart and soul to this team, and we all were better for it."
He spent the 2008 season with the Dallas Cowboys, then was in camp last year with Kansas City, which released Thomas before the season after unspecified injury issues. He didn't play again.
"The game has been good to me," said Thomas, who was joined by several friends and family, agents Drew and Jason Rosenhaus, his wife Maritza and their 4½-month-old son.
Like his final season, Thomas' stay with the Dolphins also ended after an injury.
One of the NFL's most durable players for more than a decade, Thomas appeared in just five Miami games in 2007 after his car was rear-ended, creating a whiplash injury that caused him to suffer migraines for weeks.
He also dealt with numerous concussions in his career, pledging earlier this year that, upon his death, he'll donate his brain to the Center for the Study of Traumatic Encephalopathy at Boston University School of Medicine. Thomas has often said he doesn't know how many concussions he had in his career.
On Thursday, he said he felt fine.
"I think it's great that Zach is retiring as a Miami Dolphin," Marino said in a statement. "As someone who spent my entire career with the Dolphins, I can appreciate what it means to Zach to finish his career as part of the team that meant so much to him during his playing days."
AP freelance writer Chris Perkins in Davie, Fla. contributed to this report.