Fiers, 32, has a career 42-47 record with a 4.15 ERA for the Milwaukee Brewers and Houston Astros. He led Houston with 28 starts and 153⅓ innings pitched during the 2017 regular season but was not part of the postseason roster during the Astros' World Series championship run.
Fiers became a free agent last week when the Astros declined to tender him a contract for the 2018 season.
A source said the Baltimore Orioles had also expressed interest in Fiers and offered the pitcher a two-year contract.
Fiers is the second addition to the Tigers' roster this week. The team signed outfielder Leonys Martin to a one-year major league contract Tuesday.