The Miami Herald said that releasing the 12-year veteran reduces the Dolphins' 2008 payroll by $5.7 million.
Thomas missed all but five games last season because of a concussion and post-concussion migraine headaches.
The report said he wants to continue his career but only wants to play for a contender.
"I would say Zach will not be out of work long,'' his agent, Drew Rosenhaus, told the newspaper. "I've already fielded calls about Zach. He'll play for a team that went to the playoffs last year."
The former Texas Tech standout led the Dolphins in tackles in 10 of the last 12 seasons.
His career numbers include 1,866 tackles, 18 1/2 sacks, 17 interceptions and four interception returns for touchdowns.