An NYPD detective has slapped the city with a $10 million lawsuit claiming he was discriminated against while working as a member of Mayor Bill de Blasio’s security detail because he is black.

It’s the fourth lawsuit alleging discrimination within de Blasio’s Executive Protection Unit — all filed by the same attorney, Marshall Bellovin.

In the latest lawsuit, detective Erin Fitchett claims a supervisor of the mayor’s security detail assigned him to “invisible and superfluous jobs” based on race, including shifts guarding Gracie Mansion in the freezing cold despite the fact that uniformed officers were already assigned to guard the gate.

The city’s Law Department said the Manhattan federal lawsuit is without merit.