The magazine's website quoted "sources close to the talks" and said Fiat was favoured by both GM and the German government.
A Fiat spokesman in Milan declined to comment, while Opel spokespeople were not immediately available for comment.
According to the report, however, Opel personnel oppose a deal with Fiat because they fear it would lead to job cuts. An alliance with the Italian group would lead to "massive job cuts at Opel and the closure of German factories," the head of Opel's works committee, Klaus Franz, was quoted a saying.
Both companies make similar types of cars, and Fiat, which is heavily indebted, would not be a solid partner, Franz noted. "We know the bride," he said.
GM and Fiat had a cooperation agreement that lasted five years and ended in 2005 in acrimony, a result of which GM still owes the Italian firm €1.5 billion ($2 billion), Der Spiegel said.
Just a week ago moreover, Fiat president Luca Cordero di Montezemolo rejected press reports that the group was considering a purchase of Opel. When asked if there was an Opel file in addition to one on Chrysler, the US car maker Fiat is pursuing, Montezemolo was quoted by the Ansa and Radiocor agencies as saying: "No, no there is not."
A Fiat spokesman refused to comment at the time, while an Opel spokesman had said only that there were "discussions with possible investors."