The theft took place in the early hours of Friday at the festival’s Strathallan Castle site.
Festivalgoers had been allowed into the camping area on Thursday.
Police say a “significant amount” of money was stolen along with the cash machine.
A statement said: “Police have appealed for witnesses following the theft of an ATM machine from the main arena area at T in the Park.
“The ATM contained a significant amount of cash and was taken between 12am and 7am on Friday, 8 July. Anone with information is asked to contact the police on 101.”
Meanwhile, Police Scotland have confirmed they are investigated the seperate deaths of a male and female at this year’s festival.