Relief is here for many commuters who spend hundreds on monthly transit passes.

A federal commuter tax benefit that allows transit users to pay for the cost of their commutes with up to $255 of pre-tax dollars a month is now mandatory for many employers in the five boroughs under the Commuter Benefits Law, which took effect on Jan. 1.

The law requires employers with 20 or more full-time, non-union employees to give workers access to the benefit.

The law is expected to save more than 450,000 New York City commuters up to $800 per year, according to the mayor’s office, and will benefit employers by reducing their payroll taxes.

“At a time when so many New Yorkers are struggling to stretch their hard-earned dollars, we are saving them hundreds every year,” Mayor Bill de Blasio said in a statement yesterday, after he held a press conference on the law with other elected officials and transit leaders at the Atlantic Avenue-Barclays Center subway station. The officials passed out fliers to the public to educate them about the benefit.

“NYC employees and employers alike will feel the benefits of this law in their pocketbooks,” de Blasio said.

Employers have until July 1 to comply with the law before facing possible penalties.