Kuo, 30, was having a tough spring with a 17.55 ERA in six appearances. He'd signed a one-year, non-guaranteed deal with Seattle after being released by the Dodgers following a difficult 2011 season.
If Kuo clears waivers, it's possible he could be brought back on a Minor League deal, but it was evident he was going to have a tough time cracking the 25-man roster coming out of camp.
The Mariners will need openings on the 40-man roster if they add any of the non-roster invitees remaining in camp, which is a strong possibility with infielder Munenori Kawasaki and pitcher Kevin Millwood among the seven remaining contenders.
"I think he just needs more time," said manager Eric Wedge. "You'd see times when he'd find it, but he just wasn't able to be consistent with it. And with where we are in camp and the decisions we have to make, he just wasn't going to be part of the puzzle initially."
Kuo was one of the NL's top relievers from 2008-10 with the Dodgers and earned an All-Star berth in '10, when he posted a 1.20 ERA with 73 strikeouts in 60 innings. But the native of Taiwan battled a back problem and anxiety issues in 2011, when he had a 9.00 ERA in 40 appearances before being released after the season.
The Mariners hoped the 6-foot-2, 240-pound lefty could regain his former command, but he gave up 15 hits and four walks with four strikeouts in 6 2/3 innings this spring. In his last outing, Kuo allowed four hits and four runs -- including a three-run home run -- and also hit a batter in one inning against the White Sox.
The move leaves Seattle with four left-handed relievers among 39 players in camp, with George Sherrill, Cesar Jimenez, Charlie Furbush and Rule 5 Draft pick Lucas Luetge.
The Mariners can only take 30 players when they depart for Tokyo on Thursday for their season-opening two-game series with the A's, as well as two exhibition games in Japan.