Tim Collins stood as the team's only southpaw reliever until this addition of the 27-year-old Mijares.
"We expect Jose to be a solid addition from the left side that further strengthens our bullpen," Royals GM Dayton Moore said in a statement. "He has been especially good against left-handed hitters in his career and will give [Royals manager] Ned [Yost] more options to match up late in the game."
Mijares was non-tendered by the Twins after posting a 3-6 mark with a 3.16 ERA over 186 appearances since 2008. In 2011, Mijares finished 0-2 with 4.59 ERA in 58 games.
His track record against left-handed hitters has been exceptional, with Mijares holding them to a .212 average and giving up just five home runs in 287 plate appearances. The Venezuela native currently is 3-0 with a 1.29 ERA in eight relief appearances for Tigres de Aragua in the Venezuelan Winter League.
Bringing in Mijares puts the Royals' 40-man roster at 40.