Bumgarner, 30, is a four-time All-Star and three-time World Series champion. He is also a former World Series MVP, National League Championship Series MVP and two-time Silver Slugger award winner. The 11-year veteran posted a 3.90 ERA and 9-9 record in a Major League Baseball leading 34 starts in 2019.
Bumgarner owns a 3.13 ERA and 119-92 record in 289 career appearances, all for the Giants. The No. 10 overall pick in the 2007 MLB Draft made his MLB debut in 2009.
The Giants will receive a compensation pick in next June's MLB Draft after Gardner's signing, due to the left-handed pitcher declining a one-year, $17.8 million qualifying offer from San Francisco in November. The Diamondbacks will give up a second round pick.
Neither team has publicly confirmed the deal, but the Diamondbacks posted a photo of former pitcher Randy Johnson celebrating Sunday on Twitter. The Giants responded to the tweet by tweeting a crying face emoji.