The City of New York, often called New York City or simply New York, is the most populous city in the United States and is located in the state of New York. Out-of-universe, the NYC borough of Brooklyn was one of the three main inspirations (the other two being Seattle and Portland) for the city of Hillwood, the main setting of Hey Arnold!. In-universe, New York is presumably a separate city from Hillwood, despite the two containing similarities.
This location first appeared in the episode "Married". During Helga's dream sequence, a New York City penthouse is the place she and Arnold were either living in or vacationing in after getting married in Hillwood and having their honeymoon in Venice, Italy. Various landmarks of the NYC skyline, including the Twin Towers of the former World Trade Center (the episode was produced in 2000, before the 9/11 attacks, but aired in 2002 several months after), the Brooklyn Bridge, the Statue of Liberty, the Chrysler Building, and Central Park, can be seen in the background. Also, in the episode "Rich Guy", the Statue of the Liberty and Twin Towers with the New York skyline can be seen in a background picture in Alan's room (this episode was also produced before 9/11 but aired after), alongside photos of other locations like the Grand Canyon.