A separate trial will be held to decide how much Wal-Mart should pay in damages.
More than 400 employees from 24 of Wal-Mart's 27 Oregon stores had sued the retailer, accusing it of violating federal and state wage laws.
"I guess, basically, we are disappointed with the verdict. Wal-Mart has a strong policy of paying its associates for all the time they work," said spokesman Bill Wertz. "We would emphasize that this ruling affects only approximately 350 workers out of 15,000 in Oregon."
Wertz said the company is considering whether to appeal.
The lawsuit was filed by Carolyn Thiebes and Betty Alderson, who worked in managerial positions at Wal-Mart stores in the Salem area.
The lawsuit claimed managers got employees to work off the clock by asking them to clean up the store after they had clocked out and by deleting hours from time records.
It also said Wal-Mart reprimanded employees who claimed overtime. Workers felt forced to work after clocking out because managers assigned them more work than they could complete in a regular shift, the plaintiffs said.
Wal-Mart conceded during opening arguments Tuesday that some off-the-clock work occurred but said company policy expressly prohibited it.
"What's really got to worry the company is that this might create a precedent for similar lawsuits now pending around the country - lawsuits which involve millions and millions of dollars," says CBSNews.com Legal Analyst Andrew Cohen.
"The plaintiffs in this case not only were sympathetic but they also presented a very easy to understand case to jurors," says Cohen. "Everyone who is an employee knows what it is like to be asked to work overtime, or to do a little more than the job description entails, but in most cases employees get paid for that extra work. The fact that these employees did not made Wal-Mart an easy mark."
Wal-Mart, a $218 billion company, employs 1 million workers in 3,250 stores in the United States.
Thirty-nine other class-action lawsuits are pending against the company in 30 states. Those suits, from California to New York, involve hundreds of thousands of workers seeking tens of millions in back pay.
Previously, Wal-Mart settled two similar overtime cases in Colorado and New Mexico.
The company reportedly paid $50 million two years ago to settle an off-the-clock lawsuit covering 69,000 workers in Colorado, and it recently settled for $500,000 a case involving 120 workers in Gallup, N.M., said one of the plaintiffs' attorneys.