Rodney, who will turn 37 in March, saved 85 games and posted a 1.91 ERA over the past two seasons with the Rays. He should help solidify the back end of the Mariners' bullpen.
"Rodney is signed and we're extremely happy to have him," manager Lloyd McClendon said. "He certainly solidifies the back end of our bullpen.
"We were together in Detroit. We obviously went to a World Series together [in 2006]. I know this guy. He's a tremendous frontline pitcher. He's got great character. I think the fans of Seattle are going to love him."
Seattle has two returning relievers -- right-handers Danny Farquhar and Tom Wilhelmsen -- who combined to save 40 games last season, though both had ERAs above 4.00 for the season.
Rodney's agreement could earn him up to $15 million over the two seasons if he meets incentives.
Rodney was in camp and at his locker as pitchers and catchers reported on Thursday, and he participated in drills as soon as pending blood work from his physical had cleared.
After nine years with the Tigers and Angels, Rodney had a breakout year with the Rays in 2012, when he racked up 48 saves with a 0.60 ERA, striking out 76 batters in 74 2/3 innings to earn his first All-Star Game berth, the American League Comeback Player of the Year Award and a fifth-place finish in the AL Cy Young Award voting.
He was also a key member of last year's victorious Dominican Republic team in the World Baseball Classic, saving every one of the D.R.'s victories.
Otherwise, Rodney was not as overpowering last season, but still recorded 37 saves and a 3.38 ERA with 82 strikeouts in 66 2/3 innings.
For his career, he has a 3.70 ERA and 172 saves with 551 strikeouts in 571 1/3 innings.
Rodney earned $2.5 million last year with the Rays and was regarded as one of the premier relievers on the free-agent market. Rodney said he also had offers from the Orioles, Mets and Indians, but ultimately decided on the Mariners.
"It feels good to be here and be part of a club with all the guys they have here," he said. "We have a lot of talented young guys. I might be the piece that these guys need to compete in the division. You know my situation. I try to close the game, come into the game in those situations. When the game is on the line I try to do my job. But we have the pieces here along with a good manager. The longer everybody will be together will just make it better."