Tuivailala is 3-3 with a 3.69 ERA in 31 appearances for St. Louis this season, his second full season in the big leagues. He'll immediately jump into the playoff race with Seattle holding a one-game lead over Oakland for the second wild card in the American League.
Tuivailala also gives Seattle versatility. He's worked more than one inning in eight of his 31 appearances. He's held right-handed batters to a .230 average and has not allowed a run in 21 of 31 outings.
"Sam is in the midst of his second consecutive solid big league season, and has pitched in parts of four major league seasons but has barely two years of service time,'' Dipoto said. "We view this as a move for our present and our future.''
Tuivailala will join a bullpen that's been worked extensively in numerous close ball games. Seattle is 27-12 in one-run games this season.
Elledge has quickly risen among the rankings of minor-league prospects in the Mariners organization. Elledge was recently rated as the No. 10 prospect in Seattle's organization and was 5-1 with a 1.17 ERA and nine saves in 31 games for Single-A Modesto.
Cardinals president of baseball operations John Mozeliak said Elledge will go to Double-A Springfield.
"When we're looking at where we need to be the rest of this season and into next year, we were a little nervous about guys without options and not having that flexibility as we move forward,'' Mozeliak said. "Trying to get someone like we did in that trade made a lot of sense to us in Elledge because he gives us that flexibility; he's someone we believe will move quickly.''