Cops are on the hunt for a dwarf who sexually assaulted a straphanger in Brooklyn, police said Wed­nesday.

The assailant groped a 36-year-old woman’s buttocks aboard a J train at Broadway Junction in Brownsville at about 2:45 p.m. on Jan 6, cops said.

The victim was attacked on the train by an extremely short person, police sources told The Post.

The attacker was last seen wearing white pants, an “I ♥ New York” black hooded sweatshirt, and a white cap.

An estimate of the assailant’s height wasn’t available.

But the victim described the attacker as an extremely short man wearing a baseball cap, sources said.

Although the incident took place in January, police only last night put out a call for the public’s help in finding the attacker.

Anyone with information about the incident is asked to call the NYPD’s Crime Stoppers Hotline at (800) 577-TIPS.

All calls will be kept confidential.