It was underground justice caught on video.

Good Samaritans got hold of an alleged groper and held him until police arrived to cuff his filthy paws, cops said.

Victor Sigler, 30, allegedly touched a 35-year-old woman’s butt as they left a northbound G train at Lorimer Street at about 9:45 p.m. Friday, then tried to flee by hopping a Manhattan-bound L.

But the woman — yelling, “He groped me! He groped me!” — and a pal chased after him, and some straphangers forced him off the train.

Another rider caught it all on his cellphone camera and uploaded it to YouTube — and Sigler can be seen giving his captors the finger.

He was arrested and charged with forcible touching.