"Left arm ... left elbow surgery," he told reporters.
Paxton, 32, was pulled just 1.1 innings into his first start with the Mariners last week after two years with the New York Yankees, with the injury originally described as a forearm strain.
It's been a tough two-year stretch for Paxton, with injuries limiting him to just 20.1 innings last year (1-1 with a 6.64 ERA, 1.47 WHIP and 26 strikeouts across five starts).
"I feel like for me, it's kind of been one thing after another," he told reporters on April 6. "And I work really hard and do everything that I can to be out there, and I'll continue to do so. And hopefully I can catch a break and stay healthy for a good chunk of time in the future."
He spent his first six seasons (2013-18) with Seattle before heading to New York in a blockbuster trade ahead of the 2019 campaign. He was traded back to the Mariners this offseason.
With Paxton out, Nick Margevicius will step into his place in the starting rotation, with intriguing prospect Logan Gilbert likely to get a shot as one of the team's five starters at some point this season.
The Mariners have opened the 2021 campaign 5-4 as they look to end a playoff drought dating back to 2001.