Milwaukee, which needed to replace Scott Linebrink, who went to the White Sox for $19 million over four years, still hopes to add a closer and a left fielder.
"His durability and consistency will be a great asset to our ballclub," Milwaukee general manager Doug Melvin told the Associated Press.
The signing of the right-hander comes after the departures of free agents Scott Linebrink to the Chicago White Sox and Francisco Cordero to the Cincinnati Reds.
In his career, Riske is 19-18 with a 3.40 ERA and 20 saves in 393 relief appearances with Cleveland, Boston, the White Sox and Royals.
To make room for him on the 40-man roster, the Brewers designated outfielder Laynce Nix for assignment.