Medlen's contract was purchased from Triple-A Reno on Friday.
As expected, Medlen will join the big-league squad to start Friday's game against the Astros. The 32-year-old last appeared in the majors in 2016 for the Royals, where he produced a 7.77 ERA in six starts for Kansas City. He'll face off with Gerrit Cole in his season debut. To make room for him on the active roster, Silvino Bracho was optioned to Triple-A and Taijuan Walker (elbow) was transferred to the 60-day disabled list.