Sweeney said he'd report to the A's Major League camp on Wednesday with pitchers and catchers.
"They want me to play first base and DH and be a veteran presence for the younger guys," Sweeney said from his San Diego home. "I'm excited about the opportunity and it's close to home."
Sweeney had spent his entire 17-year pro career in the Royals organization, including 13 years in the Majors. He went from backup catcher to slugging first baseman to designated hitter and team captain.
Back and knee injuries cut into his playing time in the last five years. Last season, he was limited to 74 games, batting .260 with seven homers and 38 RBIs. His back was fine but his right knee required surgery.
His future with the Royals was blocked as young Billy Butler blossomed as a big league hitter in Sweeney's DH spot and first base became crowded with candidates.
Sweeney gives the A's, whose lineup has been leaning to the left side, a boost of right-handed power. Their DH spot is manned by left-handed batter Jack Cust, who hammered 26 homers last year. Another lefty, top prospect Daric Barton, is expected to take over first base this season. He batted .347 in 18 games last September.
Sweeney's representatives had also talked with Royals general manager Dayton Moore and other clubs, including the Yankees, Rangers, Twins and Giants.
"Dayton and his staff handled it well," Sweeney said. "There was no room for me with the Royals and it was time to move on," Sweeney said.
His one-year deal with the A's, assuming he makes the Major League roster, includes incentives based on active days on the roster and at-bats, Sweeney said.
His health is good.
"This is the best I've felt in years ... and I'm hungry as I've ever been," he said.
He leaves the Royals on good terms.
"I became a Royal at age 17 and at age 34, I'm turning the reins over to the next crop of young players," he said. "I had the best years of my life in Kansas City."
In 1999, Sweeney went from being a third-string catcher who feared he'd be traded to a first baseman who blasted 22 homers, 44 doubles and drove in 102 runs. He hit .322 that year and leaves the club with a .299 career average.
His most explosive year was 2000, when he had a whopping 144 RBIs with 29 homers and a .333 average. He was an All-Star five times.
"There are only a few left in the clubhouse from when I started," Sweeney said. "There are clubhouse guys Pat Gorman and T-Man [Tom Walsh] and [coach] John Mizerock and [trainer] Nick Swartz. They're the only guys remaining."
Mizerock, who returns this year as the Royals' bullpen coach, was credited with bringing Sweeney along in the Minors.
"Without John Mizerock, I would not have made it to the big leagues," Sweeney said. "He and my dad were the biggest two influences in my career."
Sweeney will join the A's at their camp in Phoenix, not far from the Royals' complex in Surprise.
"It'll seem strange seeing the guys with me wearing a different uniform," he said. "But it's John Buck, David DeJesus, Mark Teahen, Gil Meche and Alex Gordon now -- it's their team now. And I'll be rooting for them to win a championship."
Sweeney said he'd especially miss the fans of Kansas City.
"They mean the world to me," he said.
"I'll miss wearing Kansas City blue. It's a new chapter in my life and a new chapter for the Kansas City Royals."
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.