The Mariners announced Thursday that Iwakuma's $7 million option was vested based on his on-field performance. Iwakuma missed the first month of the 2014 season but went 15-9 with a 3.52 ERA in 28 starts. In 91 career games with Seattle, Iwakuma is 38-20 with a 3.07 ERA.
General manager Jack Zduriencik said at the end of the season that Iwakuma would be returning for 2015.
Seattle also announced that left-handed pitcher Brian Moran was returned to the organization after being lost in the Rule 5 draft. Moran was selected by Toronto and traded to the Los Angeles Angels. Moran missed the entire 2014 season after undergoing Tommy John surgery on his left elbow in April.