The NYPD’s former highest-ranking uniformed officer is cashing in with his new gig at City Hall — thanks to a little-known legal loophole that allows him to collect both a pension and a taxpayer-funded salary, The Post has learned.

Retired Chief of Department Terence Monahan started collecting his $188,280 annual pension on March 28 while also earning a $242,592 annual salary as Mayor de Blasio’s new senior advisor, according to documents obtained through the Freedom of Information Law.

Retired public employees who are under the age of 65 and are collecting a pension are required to file paperwork — known as a 211 waiver — making the case for why they are uniquely qualified to collect an additional public salary.

The applications then go to the Department of Citywide Administrative Services or the New York State Civil Service Commission for approval. Without the waiver, the retiree is limited to $35,000 per year, according to state law.

But that approval was able to be sidestepped with Monahan placed on the payroll of the Economic Development Corporation — a city-funded nonprofit organization — following his February appointment to oversee the city’s safety planning for COVID recovery.

Pensioners who are employed by a public benefit corporation, like the city’s economic development arm, don’t need to file a 211 waiver, according to a 1997 decision by the city’s Law Department, DCAS spokesman Nick Benson said. The ruling has never been challenged in court.

On top of the dual income, the 60-year-old former cop is also still owed a hefty payout for unused days off, estimated at nearly $300,000, according to a review of his payroll records obtained by The Post. That lump sum payout is still pending, the city’s payroll office said.

Reached by phone recently, Monahan declined to comment on his pay.

In his new role with the EDC, Monahan is tasked with ensuring the safety of employees and visitors in the city, as well as addressing quality-of-life complaints, such as homelessness, drug use and street peddlers, with the help of an 80-cop unit in Midtown.

The 38-year police force veteran was appointed to the lucrative gig days before he was grilled by CCRB investigators over complaints of aggressive tactics used by cops during the George Floyd protests last summer.

At the same time, New York Attorney General Letitia James revised her lawsuit against the NYPD for its handling of the demonstrations, alleging Monahan “actively encouraged and participated in [NYPD’s] unlawful behavior,” including by “kettling” protesters during the chaotic Mott Haven protests.

Monahan denied that the timing of his new job had any connection to the CCRB probe or the AG’s lawsuit. The mayor has also previously rejected any link.

Police sources decried his EDC appointment as the latest example of “white-shirt immunity” — a longstanding complaint that high-ranking supervisors often get parked in cushy gigs instead of facing consequences for alleged misconduct.

“When he took that knee, he probably already knew about his next high-paying gig,” said one police source, while complaining about the “double-standard treatment that comes from 1PP.”

The source was referring to Monahan kneeling in solidarity with Black Lives Matter protesters last June as he begged for protests to remain peaceful.

Meanwhile, a city employee said people within City Hall are “frustrated” by Monahan’s new role.

“No one in City Hall wants the guy who is responsible for Mott Haven walking the halls of City Hall,” the worker said.

In a statement, DCPI spokeswoman Sgt. Jessica McRorie praised Monahan’s tenure — saying he brings a lot to the table at EDC.

“Former Chief of Department Terry Monahan spent nearly 40 years in the NYPD and led the city’s groundbreaking neighborhood policing strategy, as well as the department’s historically successful CompStat,” she said. “His vast public safety and leadership experience made him the right person to bring city agencies together, including the New York City Police Department, to help ensure recovery from the pandemic.”

A City Hall spokesperson declined to comment.