After three years with the Silverstone-based Formula One marque, a decision was taken by co-owner Vijay Mallya not to renew the 27-year-old Scot's contract as they instead opted for Sergio Perez and Nico Hulkenberg.
Although it is understood Di Resta could have driven in IndyCar for this year, the series in which cousin Dario Franchitti was seriously injured in an accident last season, he has instead opted for another crack at DTM.
The 2010 champion, who has rejoined Mercedes, said: "It feels really good to be back in the DTM and to see so many familiar faces again.
"The DTM is a fantastic racing series with three strong premium brands (Audi and BMW the others), with a very competitive field and a very evenly-matched bunch of drivers.
"So I'm really looking forward to working with the whole Mercedes-Benz DTM team and being part of another championship challenge.
"Much has changed since my last DTM race in Shanghai in 2010 with new cars, a new set of rules and a third manufacturer, but what has not changed is my enormous appetite for success.
"Mercedes-Benz has given me a car I can use to fight for wins and the title, however, I realise it will not be easy.
"My competitors already have two years of experience with the new generation of vehicles, but I'll be going flat out in order to be capable of winning at the Hockenheim season opener in early May."
Mercedes last won the DTM title with Di Resta, so he is anxious to return the marque to winning ways.
"I'm coming in with the goal of winning the first race. I don't really want to give myself time to settle in," added Di Resta.
"Maybe it will take time, but certainly when I signed the contract I signed it to go out and win the championship.
"I was the last person to win a championship here for Mercedes and bringing the title back is one of the key reasons Mercedes has brought me here."