After taking a year off in 2014, Zito returned this year to the organization that drafted him and helped him develop into a star. He spent most of the year pitching for Triple-A Nashville before making three September appearances for the A's.
Zito made his second-to-last start on Sept. 26 against the Giants and former Oakland teammate Tim Hudson, who was part of the A's "Big Three" in the early 2000s with Zito and Mark Mulder.
"I'm retiring today from baseball, but I'll never be too far away from the game that made me who I am," Zito wrote.