The Indians traded the left-hander to the Mariners on Wednesday for minor league infielder Matt Lawson, who will report to Cleveland's minor league camp.
Laffey split time between Cleveland and Triple-A Columbus last season, and he bounced back and forth between the rotation and bullpen. The 25-year-old, who was once considered a rising star in the Indians' system, was in the mix to be the club's No. 5 starter this season.
In Seattle, Laffey will be reunited with former Indians manager Eric Wedge and pitching coach Carl Willis.
"With Aaron, we felt we had the opportunity to acquire a left-handed pitcher with major league experience," Seattle general manager Jack Zduriencik said in a statement. "We will bring him to spring training and give him a chance to compete for a position on our pitching staff."
Laffey went 2-3 with a 4.53 ERA in 29 games last year for the Indians, who drafted him in 2003. He was 18-21 in 79 career games with Cleveland.
Lawson, 25, began last season in the Texas organization before he was traded to the Mariners in the deal for ace pitcher Cliff Lee. Lawson batted a combined .293 in 118 games at the Double-A level in 2010.
Last season, Lawson appeared in 99 games at second base, 13 games in left field, two games at shortstop and one game in center.