"I am glad we were able to put this together and bring Jason back to the Northwest to continue his baseball career. This is a good opportunity for Jason as well as the Seattle Mariners," Seattle general manager Jack Zduriencik said in a statement.
"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," he said.
The Mariners and Bay reached agreement on a one-year deal earlier in the week at the baseball winter meetings.
Bay was a college star at Gonzaga and lives in the Seattle area during the offseason. His career has floundered the past three seasons with the New York Mets.
Bay hit just 26 homers and played in only 288 games in his three years in New York due to injuries. He was a three-time All-Star in previous stops with Pittsburgh and Boston, and is a low risk move for the Mariners, who are looking at all avenues to try and upgrade one of the worst offenses in baseball.
Bay's contract with the Mets was terminated last month, but he is still getting paid the $21 million he was owed.
"Jason is a gritty, talented player with a winning attitude," Seattle manager Eric Wedge said. "We know he has a passion for the game and looking forward to having him on the field."