Authorities said the victim went willingly into the alley on 43rd Street in Ditmars with the suspect, though it wasn't clear if they knew each other.
The victim was taken to Elmhurst General Hospital after the assault; the suspect was gone by the time police arrived. Surveillance video shows a person believed to be the suspect fleeing the area.
He's described as being in his mid 20s, and about 6 feet tall. He was last seen wearing a white T-shirt, jeans, black sneakers and dark gloves.
Police ask anyone with information about the assault to call Crime Stoppers at (800) 577-TIPS.