In the trade, the power-hitting Morse -- who has averaged 24.5 home runs and 78.5 RBI over the last two seasons with the Nats -- was sent to the Mariners, the team he broke in with in 2005.
Seattle, meanwhile, dealt catcher John Jaso to the A's while Oakland sent three prospects to the Nationals, led by 6-foot-4 right-handed pitcher A.J. Cole.
The 30-year-old Morse, who battled injuries last season, batted .303 with 31 home runs, 36 doubles and 95 RBI over 146 games in 2011.
Jaso, 29, compiled a .276 average with 10 homers and 50 RBI for the Mariners last season.