Cishek, a side-arm-throwing, sinkerballing reliever, spent the 2021 season with the Los Angeles Angels, making 74 appearances with a 3.42 ERA and a record of 0-2.
He brings some much-needed, late-inning veteran experience to a Nationals bullpen that has been torn down as part of the team's rebuild.
Washington is coming off a 65-97 record and last-place finish in the National League East. General manager Mike Rizzo began a serious reconfiguration of the roster around the trade deadline, sending away players such as Max Scherzer, Trea Turner and Kyle Schwarber.
Cishek, 35, is 32-39 with 132 saves and a 2.85 ERA in 12 major league seasons. He has also played for the Marlins (2010-15), Cardinals (2015), Mariners (2016-17), Rays (2017), Cubs (2018-19) and White Sox (2020).
The Athletic first reported on Cishek's signing.
Information from The Associated Press was used in this report.