He'll receive a non-roster invite to spring training, according to MLB.com's Jeffrey Flanagan.
Rosenthal will earn $2 million if he makes the major-league team and can earn an additional $2.25 million in performance bonuses, reports MLB Network's Jon Heyman.
The 29-year-old struggled through a tumultuous season in 2019, authoring a 13.50 ERA in 22 appearances split between the Washington Nationals and Detroit Tigers.
Before missing the 2018 campaign while recovering from Tommy John surgery, Rosenthal was an elite closer, saving 121 games during six seasons with the St. Louis Cardinals. The 2015 All-Star owns a career ERA of 3.46 alongside a 1.36 WHIP, 2.79 FIP, and 12.0 K/9.
In Kansas City, Rosenthal will be reunited with manager Mike Matheny, whom the Royals hired as manager in October. Matheny managed the Cardinals from 2012-18.
If Rosenthal can win a job out of spring training, he'll join a Royals bullpen that finished 2019 with one of the highest combined ERAs (5.07) in the American League.