The Padres have not confirmed or commented on the signing.
Guthrie, 36, pitched inconsistently this spring after battling back spasms early in camp. He was battling A.J. Griffin for the Rangers' fifth-starter slot and allowed four earned runs in seven innings over three appearances.
The 12-year veteran went 8-8 with a 5.95 ERA over 30 appearances (24 starts) for the Royals last season, when he allowed an American League-high 29 home runs. He did not appear in any of the Kansas City's postseason games.
In San Diego, Guthrie could serve as insurance in case youngsters Robbie Erlin and Colin Rea stumble. Both have been battling for the final two spots in the Padres' rotation behind Tyson Ross, James Shields and Andrew Cashner.
Joe Trezza is a contributor for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.