"It was time to move on,'' A's general manager David Forst said.
"The at-bats just weren't there.'' Said manager Bob Melvin: "We wish him the best, but it just never ended up being a great fit here.''
Butler acknowledged frustration with a diminished platoon role in Oakland after playing every day most of his career. He believes he still has a lot to offer.
"You don't have the type of at-bats I have in the big leagues with the type of success I have for not being able to hit both sides,'' Butler said. "I feel I can bring that to a team at a championship level and I've proven that.''
Butler made headlines in August when he suffered a concussion in a fight with teammate Danny Valencia.
The San Francisco Chronicle reported that the fight began before batting practice on Aug. 19 over what Butler apparently said to Valencia while an equipment representative was in the clubhouse.
According to the Chronicle, Butler interrupted Valencia's conversation with the equipment rep to say Valencia regularly uses spikes that are in breach of his contract, adding that the rep's company should terminate Valencia's endorsement deal.
Words were exchanged between the two players, and Valencia punched Butler in the head during a brief fight, the Chronicle reported. Butler ended up on the seven-day concussion disabled list.
Butler is a career .289 hitter with 146 homers and 724 RBIs in 10 seasons. The 2012 All-Star signed a three-year, $30 million contract with the A's as a free agent in 2014 after eight seasons with the Royals.
Last year he hit .251 with 15 homers and 65 RBIs.
Information from The Associated Press was used in this report.