Hart was in the starting lineup for Friday night's game at the Chicago White Sox, but Smoak was sent down to Triple-A Tacoma. The Mariners also designated outfielder Cole Gillespie for assignment.
Hart had been sidelined by a strained left hamstring. He hit .209 with five homers in 37 games before he went on the DL in May.
Manager Lloyd McClendon says Hart also could play first base or in the outfield.
Smoak hit .208 in 63 games before straining his left quad. He hit .204 in 13 games during a rehab assignment with Tacoma.
McClendon says Smoak will eventually return as a starter with the Mariners.