The move was made after the first game of a doubleheader against the Seattle Mariners, which the Nationals lost 6-4, on Wednesday.
Rainey, a right-hander, last pitched in a game on Sunday, when he threw two scoreless innings in an extra-inning loss to the Atlanta Braves.
The Nationals recalled veteran reliever Tyler Clippard from Triple-A Rochester in a corresponding move. The 37-year-old right-hander was 4-1 with a 2.48 ERA and 49 strikeouts for Rochester in 36⅓ innings. A veteran of 15 major league seasons, he is 56-48 with 74 saves and a 3.13 ERA in his career, which includes a previous seven-year period with the Nationals in which he was a two-time All-Star.
Rainey, 29, had 12 saves this season and was 1-3 with a 3.30 ERA and 36 strikeouts in 30 innings pitched for the Nationals, who are in last place in the NL East standings.