The Athletics selected Hartlieb's contract from Triple-A Las Vegas on Wednesday.
Hartlieb has authored a 6.08 ERA and 6:9 K:BB over 13.1 innings covering eight outings with the Athletics this season, but he's pitched well in a tough environment in Las Vegas, posting a 3.35 ERA and 53:15 K:BB in 48.1 frames. He'll give the A's a fresh bullpen arm.