The team announced the deal Wednesday. Terms were not disclosed.
Mijares went 3-6 with a 3.16 ERA in 186 appearances since making his debut for the Minnesota Twins in 2008, but he was just 0-2 with a 4.59 ERA in 58 games last season.
Mijares is currently 3-0 with a 1.29 ERA in eight outings for Tigres de Aragua in the Venezuelan Winter League, with seven strikeouts and no walks.
Royals general manager Dayton Moore envisions Mijares as a situational lefty. Mijares fills out the Royals' 40-man roster.