"We don't comment on speculation," GM chairman and chief executive Dan Akerson told reporters.
"Opel is not for sale," he added, speaking at a brief news conference at a GM assembly plant in Detroit after officially opening discussions on a new labor contract with the United Auto Workers union.
Akerson also said GM, the United States's largest automobile manufacturer, would report its sixth consecutive quarterly profit next week.
The new GM that emerged from government-sponsored bankruptcy reorganization in 2009 is "not only successful but wildly successful," he said.
Rumors have been circulating in the German press for several weeks that GM might be willing to sell Opel if a buyer could be found.
Chinese companies and even Volkswagen AG, which is sitting on more than 19.5 million euros ($28.3 million) in cash, have been mentioned as possible buyers.
In 2009, GM agreed to sell Opel to a consortium led by Canadian auto-parts manufacturer Magna before changing its mind and saying it would seek to turn the brand around.
Opel is at the heart of GM's European operations, which have lost more than $14.5 billion over the past decade despite efforts at restructuring.
In addition, Opel's management team has been at odds with the company's top management in Detroit over the decision to turn Chevrolet into a "global" brand similar to Volkswagen, Toyota and Ford, GM executives have said.
The Chevrolet decision has limited Opel's access to fast-growing markets such as China and Brazil where the Opel brand has a foothold.
Tim Lee, GM's top executive in Asia-Pacific, has called Opel a "marginal" brand in Europe.