Atlanta selected Thomas' contract from Triple-A Gwinnett on Tuesday.
The left-hander made his season debut for Gwinnett in late May after completing his rehab from internal brace surgery on his elbow, and he'll now get a look in the majors. Thomas has pitched well at Triple-A with a 1.14 ERA, 0.91 WHIP and 16:7 K:BB across 23.2 innings and should fill a low-leverage relief role while up with Atlanta.