Morse was dealt from the White Sox to the Seattle Mariners on June 27, 2004. The Mariners also received Miguel Olivo and Jeremy Reed in exchange for Ben Davis and Freddy Garcia. He made his major league debut on May 31, 2005. However, four months later, Morse was suspended 10 days for violating baseball's PED policy. The 6-foot-5, 245-pound shortstop apologized to fans, teammates, the Mariners organization, and to his family. He explained that the violation occurred in 2003 when he was 21 years old and playing in the minor leagues.