The Marlins selected Ralston's contract on Tuesday and optioned him to Triple-A Jacksonville, Kevin Barral of FishonFirst.com reports.
Ralston earned a spot on the 40-man roster after posting a 1.31 ERA and 70:26 K:BB over 55 frames. The 28-year-old will remain in Jacksonville for now, but he could get an opportunity with the big club before the end of the season.