He described the Eisenach plant in eastern Germany as "a significant resource for the production of [Opel] Corsas for the long term," and said the plant was "very competitive".
'No final decisions'
Reilly indicated earlier this week that two other German plants, in Bochum and Kaiserslautern, will remain open and on Wednesday made clear that another German facility, in Ruesselsheim, is also safe.
But he said that the future of an Opel plant in Belgium was uncertain as the company moves to restructure its European operation.
After a meeting with employee representatives from Opel and sister brand Vauxhall, Reilly said that the plan will involve the total workforce being reduced "by approximately 9,000 people", but said "no final decisions" had been made.
Opel employs around 45,000 people in Europe, about 25,000 of them in Germany.
Reilly said Opel needs around $5bn of funding in the coming two years.
GM wants European countries where it also has factories to provide the money, but it said on Monday that it would restructure the auto maker without state help if needed.
The company shocked Germany and other European countries earlier this month by abruptly cancelling the planned sale of a majority in Opel to aconsortium of Canadian car-parts maker Magna International and Russian lender Sberbank.