The Nationals selected Hellickson's contract before Monday night's game at Citi Field.
Washington signed the 31-year-old Hellickson late during spring training. He was a combined 8-11 with a 5.43 ERA last year for Philadelphia and Baltimore. He has a career record of 69-69 in eight big league seasons.
The Nationals optioned righty Trevor Gott to Triple-A Syracuse. He was 0-0 with a 5.40 ERA in five games for Washington.
Catcher Jhonatan Solano was put on the 60-day disabled list because of bone chips in his right elbow. He spent a short time with Washington this season and didn't get into a game before going on the 10-day DL.