The Rays selected Johnson's contract from Triple-A Durham on Saturday, Ryan Bass of Rays.tv reports.
The Rays filled the spot in their bullpen vacated by Garrett Cleavinger, who was placed on the 15-day injured list due to left forearm tightness. Johnson has made 21 appearances (10 starts) in Triple-A this season, where he has a 3.03 ERA and 0.97 WHIP over 62.1 innings. His first appearance for the Rays will officially mark his major-league debut.