The 27-year-old appeared in 13 games last season with the Los Angeles Angels as he continued to work his way back from the Tommy John surgery he underwent during the 2018 season. Middleton has appeared in just 24 major league games over the past two seasons.
Middleton was 0-1 with a 5.25 ERA last season. His best professional season came in 2017 when he went 6-1 with a 3.86 ERA in 64 appearances for the Angels. Middleton's signing came a day after the Mariners acquired Rafael Montero from the Texas Rangers
Middleton grew up in the Portland, Oregon, area and played baseball and basketball at Lane Community College in Eugene, Oregon.
To clear a spot on the 40-man roster, the Mariners designated for assignment outfielder Phillip Ervin.