Authorities say the man in the video kicked and punched the 78-year-old victim in the face and body, causing her to bleed.
It happened just after 3 a.m. aboard a northbound 2 train on March 10.
NYPD Chief of Detectives Dermot F. Shea announced in a tweet that the person wanted for the attack was in police custody, but his identity has not been released. No charges were announced.
He also said the victim was treated and released from the hospital and "is getting the care, advocacy (and) support needed."
Officials say that equally as troubling as the incident is that no one else on the train attempted to help the woman during the incident. Instead, several of them recorded it on their phones.