New York State Route 25B (NY 25B) is a 7.25-mile (11.67 km) east–west state highway located on Long Island in New York, United States. The western terminus of the route is at an intersection with NY 25 in Queens. The eastern terminus is at an interchange with NY 25 in Westbury, Nassau County. NY 25B is named Hillside Avenue for its entire length, except for a 0.69-mile (1.11 km) portion in the village of East Williston, where it is called East Williston Avenue and owned & maintained by Nassau County as the unsigned County Route 85.

Nassau County is a suburban county located immediately to the east of New York City. As of the 2020 United States census, Nassau County's population was 1,395,774, making it the sixth-most populous county in the State of New York, and reflecting an increase of 56,242 (+4.2%) from the 1,339,532 residents enumerated at the 2010 census. Its county seat is Mineola, while the county's largest town is Hempstead. The county is part of the Long Island region of the state, lying in its southeastern portion along with the remainder of the island.