The Washington Nationals traded Morse to the Seattle Mariners on January 16, 2013, in a three team deal sending catcher John Jaso from Seattle to the Oakland Athletics and minor-league pitchers A. J. Cole, Blake Treinen and a player to be named later (Ian Krol) from Oakland to Washington.
Morse was the Opening Day left fielder, but ended up getting most of the starts in right field until the end of May. Morse hit eight home runs in spring training and four home runs in the first four games of the season.
Also, in the first thirty games, Morse hit nine home runs.
He was the first Mariner to do that since Mike Cameron in 2002.

On May 28, Morse was starting in right field against the Padres, but left after four innings, tweaking his right quad while trying to score from first on a double.
After the injury, which caused him to miss eight games, he appeared more at designated hitter and first base while trying to limit his running.
On June 22, Morse was placed on the disabled list with the injury, after he felt pain while pinch-hitting against the Angels two days prior.
On July 29, Morse was activated off the disabled list, and he started in right field the next day.
He was used mostly at right field until his trade.
In 76 games with the Mariners, Morse hit .226/.283/.410 with 13 HR and 27 RBI.
The Washington Nationals traded Morse to the Seattle Mariners on January 16, 2013, in a three team deal sending catcher John Jaso from Seattle to the Oakland Athletics and minor-league pitchers A. J. Cole, Blake Treinen and a player to be named later (Ian Krol) from Oakland to Washington. Morse was the Opening Day left fielder, but ended up getting most of the starts in right field until the end of May. Morse hit eight home runs in spring training and four home runs in the first four games of the season. Also, in the first thirty games, Morse hit nine home runs. He was the first Mariner to do that since Mike Cameron in 2002.
On May 28, Morse was starting in right field against the Padres, but left after four innings, tweaking his right quad while trying to score from first on a double. After the injury, which caused him to miss eight games, he appeared more at designated hitter and first base while trying to limit his running. On June 22, Morse was placed on the disabled list with the injury, after he felt pain while pinch-hitting against the Angels two days prior. On July 29, Morse was activated off the disabled list, and he started in right field the next day. He was used mostly at right field until his trade. In 76 games with the Mariners, Morse hit .226/.283/.410 with 13 HR and 27 RBI.
Michael Morse