To make room on the 40-man roster, left-handed pitcher Mauricio Robles was designated for assignment. The club will have 10 days to trade, release or outright Robles to the Minor Leagues.
Robles, 23, was acquired in 2009 along with Luke French in a midseason trade with the Tigers for Jarrod Washburn. He has yet to reach the Majors and split last year with Double-A Jackson and Triple-A Tacoma, posting a 5.78 ERA in 43 appearances.
Bay, 34, was a three-time All-Star with the Pirates and Red Sox and averaged 29 home runs and 99 RBIs in a six-year stretch from 2004-09. But he struggled badly the past three seasons after signing a four-year, $66 million deal with the Mets in 2010 and was released this offseason with $21 million still owed.
The Mariners signed him for $1 million in base salary, with $500,000 guaranteed and the other $500,000 coming if he makes the team out of Spring Training. The deal also includes a potential $2 million in incentives.
"I am glad we were able to put this together and bring Jason back to the Northwest to continue his baseball career," Mariners general manager Jack Zduriencik said. "This is a good opportunity for Jason as well as the Seattle Mariners. Jason has a history of being a productive Major League player and has participated in postseason play. We look forward to adding his experience to our young group of players."
Bay lives in Kirkland, Wash., less than 10 miles from Safeco Field, and attended Gonzaga University after growing up in Trail, British Columbia.
The Mariners hope Bay can regain some of the success he had prior to signing with the Mets as a high-profile free agent.
He earned an American League Silver Slugger Award in 2009 after hitting a career-high 36 home runs and 119 RBIs with Boston and was the 2004 NL Rookie of the Year with the Pirates. Bay hit 21 or more homers in six straight years, including four seasons of 30-plus home runs, 100-plus RBIs and 100-plus runs, before joining the Mets.
He hit .306 with three home runs and nine RBIs in 14 playoff games with the Red Sox and also batted .467 in five World Baseball Games for Canada in 2006 and '09.
The Mariners are looking for some veteran run producers to help a young lineup that finished last in the AL in scoring for a fourth straight year in 2012. Zduriencik is open to adding two outfielders to the mix this offseason and Bay provides one option as a right-handed-hitting corner outfielder with power potential.
"Jason is a gritty, talented player with a winning attitude," manager Eric Wedge said. "We know he has a passion for the game and we're looking forward to having him on the field when we get things going in a few months in Peoria."
The Mariners will look at Bay both as an outfielder and designated hitter. The 6-foot-2, 211-pounder played just 70 games last year while batting .165 with eight home runs and 20 RBIs. In his three years with the Mets, he hit .234 with 26 home runs and 124 RBIs.
Bay's contract buyout with the Mets gave him total free agency in exchange for deferred money. His new contract with the Mariners is exclusive of the money owed from the Mets, so whatever he makes will go on top of the remaining $21 million.
As a right-handed hitter, Bay could benefit from the left-center field alley being moved in next year at Safeco Field, though the Mets also brought in their fences a year ago at Citi Field.
He's had limited exposure to Safeco so far in his career, with a .227 batting average, one home run and two RBIs in 26 at-bats in six games.
Bay has almost exclusively played left field in his 10 years in the Majors. He joins a Mariners outfield that includes Michael Saunders, Franklin Gutierrez, Casper Wells and Eric Thames as the primary returners.
Mike Carp was targeted as the starting left fielder last year, but injured his throwing shoulder in the season opener and could also be in the mix if his shoulder has fully recovered, though he may be viewed more as a first baseman.