"Trumbo gives us another proven, middle-of-the-line-up bat," general manager Jack Zduriencik said. "He brings power and experience to the club. We think his presence will stretch and deepen our lineup with a legitimate Major League hitter."
The Mariners also acquired left-hander Vidal Nuno.
Trumbo is batting .259 with nine home runs and 23 RBIs in 46 games for the Diamondbacks this season. He has started 40 games in right field and one at first base.
His acquisition should allow Seattle the opportunity to use Nelson Cruz at designated hitter more and help balance the Mariners' left-handed heavy batting order.
Seattle's offense needs a lift thanks to the underperformance of several key players this season. Star second baseman Robinson Cano has a .627 OPS, while catcher Mike Zunino (.613) and outfielders Austin Jackson (.665) and Dustin Ackley (.546) also have been disappointing. The Mariners rank last in the American League with a .238 batting average, and they're 14th in runs scored with 191.
For the Diamondbacks, the trade helps free up at-bats for young hitters Yasmany Tomas and Jake Lamb, improves the bullpen, and provides help at catcher -- a trouble spot for the team all season. Arizona also realizes some cost savings with the deal. Trumbo is making $6.9 million this season and could be on track to earn about $10 million through salary arbitration in 2016.
Trumbo, 29, entered the major leagues in 2010 with the Los Angeles Angels before being traded to the Diamondbacks prior to the 2014 season.
He was selected to the AL All-Star team and Home Run Derby in 2012.
ESPN.com's Jerry Crasnick and The Associated Press contributed to this report.