The deal is pending a physical. Terms weren't immediately available.
Following a series of injury-plagued seasons, Buchholz became arguably the Diamondbacks' best pitcher after signing with the team in May of last season, going 7-2 with a 2.01 ERA in 16 starts.
He used a more targeted approach to revitalize his career and become a key cog in Arizona's ultimately fruitless push to a second straight playoff appearance and was eventually shut down with a flexor strain in his right arm.
The right-hander burst onto the baseball scene in 2007, tossing a no-hitter for the Boston Red Sox in his second career start. Buchholz made it through two more starts his rookie season before being shut down for shoulder fatigue. That began a string of injuries, including a pulled hamstring after making the All-Star team in 2010, a stress fracture in 2011 and a knee injury in 2014.
Buchholz then suffered an elbow injury in 2015 that bothered him over the next two years before forearm surgery cost him most of the 2017 season with Philadelphia. He ended up signing a minor league deal with Kansas City last season, but the Royals wanted to go young and kept him in Triple-A Omaha, so he opted out of the contract.
His future uncertain, Buchholz signed with the Diamondbacks, believing he could contribute but not knowing exactly what to expect.
Early in his career, Buchholz dominated hitters with a fastball that hit 96 mph. The velocity isn't quite the same now -- it's still in the low 90s -- but Buchholz has made up for it by learning hitters' tendencies, mixing his pitches and locations. He also leaned on pitching strategist Dan Haren, a former big league pitcher, and Arizona's catchers to formulate a strategy for facing opposing hitters.
News of the agreement between Buchholz and the Blue Jays was first reported by MLB.com.