An off-duty NYPD sergeant shot himself in the head in his Bronx apartment — and was found dead when his girlfriend returned home Thursday, police sources said.

The motive for the shooting, which was reported to 911 shortly after 2 p.m., was unclear.

The dead cop, who hasn’t been officially identified, joined the NYPD in 2010.

He was assigned to the NYPD Housing Bureau’s Public Service Area 7, which patrols public housing projects in the South Bronx, sources said.

Those projects include the infamous Mott Haven Houses, where two cops were shot and wounded by a gunman inside a stairwell while Mayor Bill de Blasio was delivering his State of the City speech at the Bronx’s Lehman College last year.