New York State Route 231 (NY 231) is a north–south state highway located in Suffolk County, New York, in the United States. The route extends for 9.05 miles (14.56 km) from a partial interchange with NY 27A in Babylon to an interchange with the Northern State Parkway in Dix Hills. The southernmost 2 miles (3.2 km) of NY 231 is a limited-access highway known as the Babylon–Northport Expressway; the remainder of NY 231 is known as Deer Park Avenue.

NY 231 begins at an interchange with Route 27A (Montauk Highway) in Babylon. The route progresses northward around Hawleys Lake as a four-lane freeway. A short distance after NY 27A, NY 231 meets ramps from County Route 50 (CR 50, named John Street), but does provide service to the county road from either direction. After John Street, the freeway crosses over the Long Island Railroad's Babylon Branch and crosses northward through a large residential district. After paralleling Madison Street in North Babylon, NY 231 intersects with NY 27 (the Sunrise Highway) in a full cloverleaf interchange. From NY 27, NY 231 heads northward and northwestward, intersecting with Hunter Avenue in North Babylon, where the freeway portion ends and the highway becomes an arterial boulevard. The route curves back to the north through the large commercial district, interchanging with CR 34 (Deer Park Avenue). Right after the interchange, NY 231 interchanges with the Southern State Parkway at a full cloverleaf in North Babylon.

After the Southern State Parkway, NY 231 continues northward as an arterial boulevard named Deer Park Avenue. The surroundings are mostly commercial, continuing as a four-lane road and intersecting with CR 4 (Commack Road) at a fork. After Commack Road, NY 231 becomes a residential arterial with a grassy, divided median and intersecting with CR 57 (Bay Shore Road) in Deer Park. The highway continues northward through Deer Park, crossing through a large commercial strip and under another Long Island Rail Road line. There is a short interchange with Long Island Avenue in Deer Park as NY 231 continues into Dix Hills as Deer Park Road. In Dix Hills, NY 231 becomes residential once again, crossing a major residential complex before intersecting with CR 2 (Straight Path).

The arterial remains residential, continuing northward into an interchange with Interstate 495 (I-495, named the Long Island Expressway) in Dix Hills. Here, the road expands to five lanes and crosses under the interstate, where NY 231 becomes four lanes once again. The highway continues northward along the residential strip, intersecting with CR 67 (the Long Island Motor Parkway). After NY 231 curves to the northwest, it enters a partial cloverleaf interchange with the Northern State Parkway, where the designation terminates. The right-of-way continues northward as CR 35 (Deer Park Road).

The entirety of Deer Park Avenue and Deer Park Road was originally designated as part of CR 35 on February 24, 1930. In the 1960s, the portion of CR 35 between the Southern and Northern State Parkways was transferred to the state of New York, which designated the highway as NY 231. Initially, NY 231 followed Deer Park Avenue (now CR 34) south to NY 27A in Babylon; however, it was realigned c. 1969 to follow a 2-mile (3.2 km) section of the new Babylon–Northport Expressway. As its name implies, the highway was to run from Babylon to Northport, a distance of 14 miles (23 km); however, no other sections of the road were constructed.

The entire route is in Suffolk County.