Martinez was called up from Triple-A Las Vegas and will start Sunday against the Yankees.
The 25-year-old was demoted to Las Vegas after giving up two runs over 4.1 innings against the Giants on Aug. 7, and he'll rejoin the A's to start in the series finale versus the Yankees. Martinez has a 6.08 ERA in the majors this year and surrendered eight runs across his two minor-league starts since facing San Francisco.