Judge Frank Roesch's ruling came after a plastic-bag industry group called the Coalition to Support Plastic Bag Recycling sued Oakland last summer shortly after the City Council approved a ban on single-use plastic bags at retail stores doing more than $1 million a year in business. The judge heard arguments in the case in January.
The ban was billed as an environmentally friendly ordinance. But at the crux of the case was a question on whether the increased use of paper bags could harm the environment as well.
Paper bags take more energy to create and fill up more landfill space, the plastic-bag industry argued.
"The court ... finds that substantial evidence in the record supports at least a fair argument that single-use paper bags are more damaging than single-use plastic bags," Roesch wrote.
City Attorney John Russo's office will ask the City Council next week whether its members want to contest Roesch's ruling or do a full environmental review of the ban, Russo spokesman Alex Katz said.
A full environmental review could cost $100,000, maybe more.
Oakland's ordinance followed a similar ban in San Francisco, and the legal dispute in Oakland was watched by cities in California and throughout the country that are weighing similar action.
Keith Christman, senior director of the American Chemistry Council's plastics division, released a statement saying the group was pleased with the judge's ruling.
"Banning plastic bags would dramatically increase energy use, double greenhouse gas emissions and increase waste," he said. "Recycling plastic bags is the right approach and makes plastic bags the environmentally responsible choice."
Throughout the legal battle, City Council members, including Jean Quan (Montclair-Laurel) and Nancy Nadel (Downtown-West Oakland), disagreed with the industry position that banning plastic bags could harm the environment.
Some grocery stores in Oakland including Trader Joe's, Whole Foods and Farmer Joe's moved away from plastic on their own.
In February, Farmer Joe's co-owner Diana Tam said, "We would like to be mindful of the plastic-bag problems that we're facing with the plastic bags going into the landfills and not disintegrating."
Wire services contributed to this report.