The Mariners acquired Dominguez, outfielder Nolan Jones and catcher Boston Smith from the White Sox on Saturday in exchange for right-hander Luis Castillo, Jeff Passan of ESPN.com reports.
Dominguez opened the season as the closer for the White Sox but lost his grip on the job in late June. He'll now head to Seattle, where he'll presumably operate in a middle relief for the Mariners, with Andres Munoz locked in as the closer. Dominguez has a 4.10 ERA, 1.37 WHIP, 12 saves (in 17 chances), four holds and a 45:23 K:BB over 37.1 innings this season.