A teen creep has been nabbed for sexually abusing a woman riding a Manhattan train with four kids and then brushing off the sick act as “my bad” — only to be cut loose by a judge shortly after his arrest, prosecutors and sources said.

Ari Shtein, 18, allegedly reached into the 30-year-old victim’s underwear and placed two fingers inside her private area as he stood behind her on board an uptown R train approaching 28th Street in Chelsea around 3 p.m. June 28, according to cops and a criminal complaint.

 “My bad, didn’t know you were going to react that way,” Shtein, who hails from Washington, DC, and does not know the victim, allegedly told her after the disturbing act, according to the sources.

Ari Shtein, 18, allegedly penetrated the 30-year-old victim’s private area with his fingers on board an uptown R train, cops and law enforcement sources said. NYPD The victim was traveling with four kids, sources said. Stefano Giovannini

The victim, who sources say was traveling with four kids — ranging in age from 7 to 11 — was taken to a local hospital for observation, police and prosecutors said

It was not immediately clear whether the kids are her biological children.

Shtein initially fled the scene, but surrendered to cops just after 7 a.m. Wednesday, police said.

He was charged with sexual abuse, aggravated sexual abuse, forcible touching and endangering the welfare of a child, prosecutors said.

Shtein was charged with aggravated sexual abuse and endangering the welfare of a child, cops said. Zandy Mangold

Shtein was released on his own recognizance during his Wednesday arraignment in Manhattan Criminal Court, over prosecutors’ request that he be held on $5,000 bail or $10,000 bond.

He has no previous arrests in the Big Apple, cops said.

His attorney did not return a call for comment from The Post.