The Twins selected Jackson's contract from Triple-A St. Paul on Wednesday.
Minnesota cleared room for Jackson on the 26-man active roster by optioning right-hander Louie Varland to Triple-A and on the 40-man roster by designating right-hander Diego Castillo for assignment. The 36-year-old Jackson is joining the Twins for the second time this season after he was included on the Opening Day roster before he was DFA'd in May after turning in a 6.85 ERA over 22.1 innings. The veteran righty turned in three scoreless one-inning appearances for St. Paul to get the call back to Minnesota.