The deal allows Ibanez to earn an additional $1.25 million in performance bonuses, the person said, speaking on condition of anonymity because the agreement had not been announced.
Ibanez returns to the team he began his big league career with from 1996-00, then rejoined from 2004-08.
Now 40, Ibanez spent the past season with the New York Yankees and became popular with fans for his late-game home runs. He had hoped to remain with New York, but the Yankees have moved slowly during the offseason.
Ibanez hit .240 with 19 homers and 62 RBIs in 384 at-bats, his pull swing making him a natural for the short right-field porch at Yankee Stadium. He batted just .197 with five RBIs in 61 at-bats against left-handers.
Including the playoffs, Ibanez hit five home runs that tied the score for the Yankees and eight that put New York ahead, according to STATS. He homered twice after entering as a pinch hitter on Sept. 22 in a 10-9, 14-inning win over Oakland. And with New York fighting for the AL East title, he delivered a tying, pinch-hit homer against Boston in the ninth on Oct. 2 and then singled in the winning run in the 12th.
Then in Game 3 of the division series against Baltimore, he became the first player in major league history to homer twice in a postseason game he didn't start. He pinch-hit for Alex Rodriguez in the ninth inning and hit a tying home run, then hit a winning shot in the 12th.
Three days later his two-run homer in a four-run ninth inning tied the AL championship series opener against Detroit, a game the Yankees lost 6-4 in 12 innings as the Tigers started their way to a four-game sweep.
Ibanez had a $1.1 million base salary last season and earned another $2.05 million in performance bonuses.
He joins a Seattle team that added power-hitting Kendrys Morales earlier in the week in a trade that sent left-hander Jason Vargas to the Los Angeles Angels. Former Yankees prospect Jesus Montero started 77 games at designated hitter last season for the Mariners and 55 behind the plate, so Ibanez's acquisition by the Mariners could make catcher John Jaso expendable. Jaso made 39 starts behind the plate and 44 at DH.
In 17 major league seasons that also included time with Kansas City (2001-03) and Philadelphia (2009-11), Ibanez has a .278 career average with 271 home runs and 1,116 RBIs.
With Ichiro Suzuki likely to see most of his time in a corner outfield spot because New York hasn't attempted to re-sign Nick Swisher, Ibanez's departure leaves the Yankees searching for a left-handed bat for a part-time designated hitter role. New York's priority before filling that spot appears to be a right-handed bat because Andruw Jones left for Rakuten in Japan after a subpar season and all three starting outfielders - Suzuki, Curtis Granderson and Brett Gardner - are left-handed hitters.
Ibanez's deal was negotiated by agents Sam and Seth Levinson.