The US Supreme Court will hear oral arguments next month in a case that could dramatically curb the power of public-sector unions — especially in New York.

It’s the third attempt to overturn a 1977 ruling that upheld the forced payment of “agency fees” to unions that represent non-members in contract negotiations.

If plaintiff Mark Janus, an Illinois child care specialist, prevails, it could cost government-employee unions $110 million a year in New York state alone, according to a new Empire Center report.

Four decades ago, the Supreme Court upheld the right of unions to charge compulsory “agency fees.” It was on the verge of overturning that ruling in 2015 when Justice Antonin Scalia died and the remaining justices deadlocked.

Now the Janus case brings a very real possibility that the high court, with Justice Neil Gorsuch aboard, will reverse the ’77 decision.

As the Empire Center’s Ken Girardin notes in the report, agency fees — part of the $862 million New York’s public unions collect each year — contribute greatly to the unions’ outsized political power.

Influence that has landed them lavish retirement and other benefits that are slowly bankrupting the state and city.

Serious First Amendment questions are in play, Girardin notes: New York public employees, unlike unionized private-sector workers, “don’t have access to information on how their union dues and fees are spent.”

Besides, should workers who don’t want to join a union be forced to pay one anyway as a condition of keeping their jobs?

Fifteen of every 100 New York jobs are with state or local government, and union contracts cover 73 percent of those public jobs, double the national average.

Clearly, the unions are worried: They’re lobbying Albany for laws to protect agency fees, and Gov. Cuomo has signaled support for the drive.

Someone should tell the gov that if the Supreme Court strikes this long-overdue blow for freedom in the workplace, it will boost New York’s competitive economic position.