The Nationals made the move before playing Saturday at Citizens Bank Park. Reliever Sammy Solis was sent to Triple-A Syracuse.
Hellickson is 2-0 with a 2.28 ERA in nine starts this season. The veteran right-hander has been sidelined since June 4 with a right hamstring strain. He was 6-5 for Philadelphia last season.
Solis is 1-1 with a 4.50 ERA in 40 games this season. In 28 innings, the left-hander has allowed 22 hits with 30 strikeouts and 15 walks.
Also, Washington transferred third baseman Ryan Zimmerman, who has been out since May 10 with a strained right oblique, to the 60-day DL.
The Nationals reinstated catcher Raudy Read from the restricted list and optioned him to Triple-A Syracuse. Read, who appeared in eight games last season, is coming off an 80-game suspension after testing positive for a performance-enhancing substance.