The Nationals selected the contract of RHP Abel de Los Santos from Double-A Harrisburg, the team announced on Monday.
De Los Santos was 4-4 with six saves and a 2.65 ERA at Harrisburg.
He replaces RHP Blake Treinen, optioned to Triple-A Syracuse. He was 2-2 with a 4.39 ERA with the Nats.
To make room on the 40-man roster, outfielder Jayson Werth was moved to the 60-day disabled list.