Maness' deal is worth $1.25 million if he makes the Majors, with $750,000 worth of performance bonuses, a source told MLB.com.
The Royals already have veterans Jonathan Sanchez, Al Alburquerque, Chris Withrow, Brandon League and Bobby Parnell as non-roster Spring Training invites in camp.
Maness, 28, will compete with those veterans for one, possibly two spots in the bullpen.
The Royals were one of several teams to scout Maness at a recent tryout in St. Louis. Maness underwent an experimental elbow surgery instead of the traditional Tommy John surgery last August, and is expected to be ready for Opening Day.
Maness had a 3.19 ERA in 244 relief outings over four seasons with the Cardinals.
Jeffrey Flanagan has covered the Royals since 1991, and for MLB.com since 2015. Follow him on Twitter @FlannyMLB. This story was not subject to the approval of Major League Baseball or its clubs.