A Harlem cop shot a gun-toting teen in the arm early yesterday — but only after the kid opened fire first, the NYPD said.

The unidentified cop spotted an armed Shawn Lambert, 17, in front of 2130 Madison Ave. at 1:30 a.m., authorities said. Lambert allegedly started shooting, and the officer fired back five rounds. A bullet struck the kid’s right arm.

He ran off, but the cop followed the trail of blood to an apartment on East 135th Street. Lambert was in stable condition last night at Harlem Hospital.