Ramirez signed a minor league deal with Oakland on Feb. 20 and hit .302 with 14 RBIs in 17 games with Triple-A Sacramento. He served a 50-game suspension for violating Major League Baseball's drug policy last year, but remained in the minors when the ban ended on his 40th birthday on May 30.
"It looked like he was down there for a while and I think he wanted out," A's manager Bob Melvin said. "He wanted an opportunity, if we weren't going to do something, to go somewhere else. It is what it is. Certainly we wish him the best."
The dreadlocked slugger was expected to make approximately $500,000 if he was added to Oakland's big league roster.
"Manny believes he has demonstrated that he is ready to return to the major leagues," Ramirez's agents, Barry Praver and Scott Shapiro, said in a statement. "However, given that the Oakland Athletics could not give Manny any assurance that they plan to promote him in the immediate future he asked for his release. Manny thanks the A's for providing him with this opportunity."
Ramirez, who has 555 career homers and 1,831 RBIs, retired last season rather than serve a 100-game suspension for failing a second drug test. But Commissioner Bud Selig cut the suspension in half after Ramirez sat out the balance of the season.
He served a 50-game suspension in 2009 while with the Los Angeles Dodgers organization, playing parts of three games in Albuquerque during his rehabilitation assignment.
The 12-time All-Star went 1 for 17 (.059) in five games last season for Tampa Bay, which had signed him to a one-year deal worth $2.02 million.