Reinhardt's decision overturned a ruling last year by a smaller panel of 9th Circuit judges that had been criticized by equal pay advocates.
"Although the (Equal Pay) Act has prohibited sex-based wage discrimination for more than fifty years, the financial exploitation of working women embodied by the gender pay gap continues to be an embarrassing reality of our economy," Reinhardt wrote.
Women made about 80 cents for every dollar men earned in 2015, according to U.S. government data.
The ruling came in a lawsuit by California school employee Aileen Rizo, who learned in 2012 while having lunch with her colleagues that male counterparts hired after her were making more money.
Fresno County public schools hired Rizo as a math consultant in 2009 for a little under $63,000 a year. The county had a standard policy that added 5 percent to her previous pay as a middle school math teacher in Arizona.
Fresno County Superintendent of Schools Jim Yovino says he will appeal the ruling in the equal pay lawsuit to the U.S. Supreme Court.
In announcing the appeal, Yovino said the policy of his office for determining salaries did not discriminate on the basis of gender.
An email to an attorney for Yovino was not immediately returned.
The Equal Pay Act, signed into law by President John F. Kennedy in 1963, forbids employers from paying women less than men based on gender for equal work performed under similar working conditions. But it creates exemptions when pay is based on seniority, merit, quantity or quality of work, or "any other factor other than sex."
Fresno County argued that basing starting salaries primarily on previous pay was one of those other factors and prevented subjective determinations of a new employee's value.