After taking a year off from baseball, Zito signed a minor league contract to return to the Athletics on February 16, 2015.
In spring training, Zito competed for a role on the Athletics' 25-man roster, possibly as a long reliever.
On April 4, 2015, Zito accepted an assignment to the Triple-A Nashville Sounds.
Zito's Nashville teammates lauded him for embracing the Triple-A lifestyle and for his commitment to the team: charting pitches between starts, coaching first base, and even buying dinner for the entire team on his birthday.
Zito spent the entire season with Nashville, including about a month on the disabled list with left shoulder tendinitis.
He was activated on the next-to-last day of the season on which he pitched one scoreless inning of relief.
In a total of 24 appearances (22 starts), he accrued an 8–7 record with a 3.46 ERA and 91 strikeouts.

Zito revealed in an interview that he had learned the Athletics would not be bringing him up to the major league club in September.
 However, following a season-ending injury to Jesse Chavez, Oakland purchased Zito's contract from Triple-A on September 16, placing him on the major league roster.
Zito made his first major league appearance of the season on September 20, pitching an inning in relief.
On September 26, 2015, Zito started for the Athletics against Hudson and the Giants in a matchup that was arranged as a tribute to the A's "Big Three" of the early 2000s.
Both pitchers received lengthy standing ovations from the sold-out Coliseum crowd (which included the third Big Three member, Mulder) upon leaving the game.

On October 19, 2015, Zito announced his retirement from baseball in an article for The Players' Tribune.
After taking a year off from baseball, Zito signed a minor league contract to return to the Athletics on February 16, 2015. In spring training, Zito competed for a role on the Athletics' 25-man roster, possibly as a long reliever. On April 4, 2015, Zito accepted an assignment to the Triple-A Nashville Sounds. Zito's Nashville teammates lauded him for embracing the Triple-A lifestyle and for his commitment to the team: charting pitches between starts, coaching first base, and even buying dinner for the entire team on his birthday. Zito spent the entire season with Nashville, including about a month on the disabled list with left shoulder tendinitis. He was activated on the next-to-last day of the season on which he pitched one scoreless inning of relief. In a total of 24 appearances (22 starts), he accrued an 8–7 record with a 3.46 ERA and 91 strikeouts.
Zito revealed in an interview that he had learned the Athletics would not be bringing him up to the major league club in September.  However, following a season-ending injury to Jesse Chavez, Oakland purchased Zito's contract from Triple-A on September 16, placing him on the major league roster. Zito made his first major league appearance of the season on September 20, pitching an inning in relief. On September 26, 2015, Zito started for the Athletics against Hudson and the Giants in a matchup that was arranged as a tribute to the A's "Big Three" of the early 2000s. Both pitchers received lengthy standing ovations from the sold-out Coliseum crowd (which included the third Big Three member, Mulder) upon leaving the game.
On October 19, 2015, Zito announced his retirement from baseball in an article for The Players' Tribune.
Barry Zito