Riske signed a three-year deal to pitch in the back end of the Brewers' bullpen after passing a physical examination.
Anticipating his departure, the Royals earlier signed Japanese setup reliever Yashuhiko Yabuta, a free agent from the Chiba Lotte Marines.
Riske last season was extremely effective for the Royals after a rocky April, finishing with a 2.45 ERA in 65 games. He also was valued as a mentor to younger relief pitchers.
The Brewers lost closer Francisco Cordero, who signed with the Cincinnati Reds, and will try Derrick Turnbow in that role. But Riske, had four saves for the Royals last season, could be a fallback option.
He is the second Royals free agent to go elsewhere. Catcher Jason LaRue signed with the St. Louis Cardinals. Other remaining KC free agents are designated hitter Mike Sweeney, pitchers Odalis Perez and John Thomson, and outfielder Reggie Sanders.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.