The Reds selected Richardson's contract from Triple-A Louisville on Saturday.
Richardson has been with Louisville all season, turning in a 4.75 ERA and 1.26 WHIP alongside a 34:14 K:BB through 30.1 innings. He'll head to the majors as a replacement for Pierce Johnson (elbow) and likely be limited to low-leverage work as long as he's with the Reds. Kyle Nicolas was designated for assignment in a corresponding move.