The lawsuit accuses Wal-Mart, the nation's largest private employer, of paying its female employees less than male workers who held the same jobs and bypassed women for promotions, the Associated Press reported.
U.S. District Judge Martin Jenkins in San Francisco certified the suit of roughly 1.6 million women who worked for Wal-Mart's U.S. stores since Dec. 26, 1998.
"I think it's a terrific victory for the women who work at Wal-Mart who have labored for years under working conditions where they have been told repeatedly they have been unsuitable for management and not suitable to make as much as men," said Joseph Sellers, one of the attorneys representing the women.
Wal-Mart has previously denied discrimination, saying its number of men in management positions is a reflection of the higher number of applications from men.
"Up until now, Wal-Mart has never faced a trial like this," Brad Seligman, lead attorney for the six women plaintiffs, said in a statement. "Lawsuits by individual women had no more effect than a pinprick. Now, however, the playing field has been leveled."
At a hearing in September, Wal-Mart attorneys asked the judge to hear the case as small class action lawsuits targeted to each store separately, saying that the stores operate as independent businesses.
Seligman responded that Wal-Mart stores are "virtually identical in structure and job duties."
"There is a high emphasis on a common culture, which is the glue that holds the company together," he said.
The "plaintiffs present largely uncontested descriptive statistics which show that women working at Wal-Mart stores are paid less than men in every region, that pay disparities exist in most job categories, that the salary gap widens over time, that women take longer to enter management positions, and that the higher one looks in the organization the lower the percentage of women," the judge decided.
So far, the case has created 1.25 million pages of evidence and 200 depositions, according to the AP.
A judge or jury must first determine if Wal-Mart is guilty of discrimination before the plaintiffs could seek damages.
Wal-Mart spokeswoman Mona Williams told the AP that the company is making changes.
"Earlier this month Wal-Mart announced a new job classification and pay structure for hourly associates," Williams said. "This new pay plan was developed with the assistance of third-party consultants and is designed to ensure internal equity and external competitiveness."