The deal is for $2.2 million with $1 million in performance bonuses. Boxberger can earn an additional $500,000 in bonuses for appearing in 60 games and finishing 45 of them.
Royals general manager Dayton Moore said last week he was open to adding help in the bullpen ahead of spring training. Boxberger could compete with Wily Peralta, who was 14 of 14 in save opportunities last season, for the right to close out games in Kansas City
Boxberger, 30, claimed the closer's role for the Arizona Diamondbacks after being acquired prior to the 2018 season, saving 32 games to go along with a 4.39 ERA. He was not tendered a contract after he earned $1.85 million last season, making him a free agent.
Prior to joining the Diamondbacks, Boxberger spent four seasons with the Tampa Bay Rays, with whom he was an All-Star and led the American League in saves with 41 in 2015.
His most dominant season came as a set-up man in 2014, when he had a 2.37 ERA and struck out 104 batters in 64 2/3 innings. After taking on the closer role in 2015, Boxberger reverted to a set-up role over the next two seasons for the Rays.
The 30-year-old Boxberger was drafted by the Royals in 2006 but went unsigned, instead pitching for USC. He was then drafted by the Reds in 2009. He has a career 3.42 ERA with 76 saves and 368 strikeouts.
The Associated Press contributed to this report.