Smith is expected to join Justin Ruggiano in a right-field platoon for the Mariners, who already had upgraded their lineup by signing free-agent outfielder Nelson Cruz to a multiyear contract earlier this offseason.
The trade is the latest in a flurry of moves for new San Diego general manager A.J. Preller, who acquired Matt Kemp, Justin Upton and Wil Myers in a series of trades shortly before Christmas and moved Smith to help ease the glut of outfielders on the Padres' roster.
Smith, 32, batted .266 with 12 homers and 48 RBIs last season. But against right-handed pitching, Smith hit .270 with all 12 of his home runs and an .815 on-base plus slugging percentage.
Seattle general manager Jack Zduriencik wouldn't call his right-field situation a platoon but said Ruggiano's numbers against left-handed pitching and Smith's success against right-handers could make for a nice mix.
"We were in search of adding another outfielder. We've got a player that's a professional hitter, a guy that we think is going to give us really good [at-bats] especially against right-handed pitching," Zduriencik said.
Seattle also gets some time out of the trade. Smith is under contract through the 2016 season with a club option for 2017.
"One of the things we try and stay away from is giving up talent for one-year returns," Zduriencik said.
Maurer, 24, originally was a starter with Seattle last season but thrived after moving to the bullpen. Maurer made 31 relief appearances and had a 2.17 ERA coming out of the bullpen. He had 38 strikeouts in 37 1/3 relief innings.
The Associated Press contributed to this report.