Garate, who will work out of the bullpen, was acquired off waivers from the Dodgers on Wednesday after posting a 2.04 ERA and a .191 opponents' batting average in 47 appearances with Double-A Chattanooga this season.
Garate's first appearance with the Nationals will mark his big league debut. He will wear No. 56.
Interim manager Jim Riggleman said the team would be careful when it comes to using Garate, because he hasn't pitched in a week. Garate was waiting to find out if he was going to leave the Dodgers.
"We want to see him get an inning or two and then take a day off," Riggleman said. "We'll run him out there again and see what we have there. By the end of September, we'll know if he is in the mix for the future."
Garate, 24, is 23-7 with 15 saves and a 3.25 ERA in five professional seasons. With 404 strikeouts in 315 1/3 innings, Garate has posted 11.53 strikeouts per nine innings during his career. He was rated among the Dodgers' top 25 prospects by Baseball America entering the 2009 season.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.