Mejia has a .273 career batting average with 49 stolen bases, a home run and 42 RBI in 133 minor league games in the Seattle organization after being signed as an amateur free agent in June 2012 out of the Dominican Republic. The 21-year-old switch-hitter combined to hit .282 in 51 games with Triple-A Tacoma, Single-A Clinton and Everett and the rookie level AZL Mariners last season.
Wieland spent most of last season with Triple-A Oklahoma City, going 10-5 with a 4.59 ERA in 22 games. The 25-year-old right-hander also made two starts for the Dodgers, with a 0-1 record and an 8.31 ERA. He was originally acquired by Los Angeles from San Diego in December 2014 in a five-player deal.
The deal announced Tuesday leaves the Dodgers with 39 players on their 40-man roster.
To make room on their roster, the Mariners designated right-hander A.J. Schugel for assignment.
In another deal, the Dodgers acquired minor league infielder-outfielder Rob Segedin and a player to be named or cash considerations from the Yankeesfor infielder Ronald Torreyes and left-hander Tyler Olson.
Last week, the Dodgers designated Torreyes and Olson for assignment.
Segedin combined to hit .287 with seven home runs and 34 RBI in 71 games last season with the Yankees' Triple-A Scranton-Wilkes-Barre and Double-A Trenton. The 27-year-old has a .272 career batting average in 458 games in six minor league seasons in the New York organization. He's mostly played third base, but has also seen action in right field, first base and left field.