A new law in California forces companies to prove higher salaries paid to men are based on factors other than gender.
On Tuesday, California Governor Jerry Brown signed into law 'Fair Pay Act' expanding wage discrimination laws in the state.
Supporters called it the strongest equal pay regulation in the US.
California companies will have to show that higher salaries earned by men are based on experience or skill alone.
Businesses accused of wage discrimination will have to demonstrate men who are paid more receive their wage based on any "bona fide factor other than sex".
Companies can show a male employee has seniority, is ranked higher on a merit based system or performs a greater quantity or quality of work.
Senator Hannah Beth Jackson, who wrote the bill, told the BBC's World Service: "After 35 years of fighting for equal pay we have now the strongest law in the entire country."
Noreen Farrell, executive director of Equal Rights Advocates which sponsored the legislation told the BBC: "We engaged the business community on this bill so they could justify pay difference for reasons that aren't gender related and that's what brought us a bill both business and workers can get behind."
But, she acknowledged the law is only one step.
"There are really great provisions to allow people to talk about pay, but there isn't absolute transparency on pay in offices that aren't run by the government or that don't have a union," she said.
The law comes into effect on January 1.
Companies will not have to make public how much each employee earns, but will be subject to harsher penalties if they retaliate or discriminate against individuals who ask or share this information.
The California Chamber of Commerce applauded the bill.
In a statement the Chamber's president Allan Zaremberg said: "Equal pay for equal work, regardless of gender, shouldn't be an issue in California. We applaud the Governor and a bi-partisan vote in the legislature for establishing this fundamental tenet in statute and providing guidance to employers to determine appropriate wages for non-gender related reasons that allow employers to effectively manage their workforce."
In 2013 in California women made $0.84 cents (£0.57) for every $1 (£0.65) earned by a man, according to Equal Rights Advocates.
In 2009 President Barack Obama signed the Lilly Ledbetter Fair Pay Act to fight pay discrimination, but has not been able to make much headway since then.