Sources told ESPN that Estrada and the A's reached an agreement on a one-year deal worth $4 million.
The right-handed pitcher adds veteran stability to the Athletics' starting rotation. Prior to the acquisition of Estrada, Oakland's projected rotation included Mike Fiers, Jesus Luzardo, Daniel Mengden, Chris Bassitt and Paul Blackburn.
Estrada arrives in Oakland after back-to-back disappointing seasons with the Toronto Blue Jays. He posted a 5.64 ERA across 28 starts in 2018 and a 4.98 ERA in 33 starts during his 2017 campaign.
The A's hope Estrada can regain the form he had in the 2015 and 2016 seasons. The one-time All-Star averaged a 3.30 ERA during that span.
"Last year, he pitched through some back and hip issues and lost a little bit of velocity," A's general manager David Forst said in a statement. "But his command is as outstanding as ever. His changeup has always been one of the best pitches in the league. He's a guy we fully expect to bounce back and be near the top of our rotation for 2019."
Estrada also gives the A's postseason experience, compiling a 3-3 record and 2.64 ERA in 10 playoff games (six starts).