New York State Route 111 (NY 111) is a state highway located in Suffolk County, New York, in the United States. It runs north–south for 9.42 miles (15.16 km) through the towns of Islip and Smithtown, connecting NY 27A in the town seat of Islip to the east end of the NY 25 and NY 25A concurrency in Smithtown's Village of the Branch. Most of the road is a two-lane highway, save for several short four-lane stretches in the vicinity of interchanges along the route. NY 111 was assigned as part of the 1930 renumbering of state highways in New York as an East Islip–Village of the Branch highway, using what is now County Route 17 (CR 17) south of Central Islip. It was realigned to serve the hamlet of Islip in 1966.

NY 111 begins at an intersection with NY 27A (Main Street) in the town of Islip. The route proceeds north along Islip Avenue, crossing through a short residential stretch before the intersection with CR 50 (Union Boulevard). NY 111 crosses the Montauk Branch of the Long Island Rail Road just east of the Islip station, continuing north past various businesses and residences along the two-lane roadway. Continuing north, NY 111 intersects with Bittermint Street, which connects to NY 27 (Sunrise Highway) eastbound. After crossing over NY 27, the route intersects with a ramp to serve the westbound lanes and Islip Boulevard, which connects NY 27 westbound to NY 111.

NY 111 continues northward along Islip Avenue north of NY 27, crossing through blocks of residences in Islip. Just north of Spur Drive, the route enters a cloverleaf interchange with the Heckscher State Parkway (exit 43). After the Heckscher State Parkway, NY 111 continues north through Islip as a two-lane residential street entering Central Islip. Further into Central Islip, the residences give way to various businesses, bending northwest on Islip Avenue. After several blocks, this change reverts itself, crossing over another Long Island Rail Road line, this time the Ronkonkoma Branch. NY 111 then intersects with CR 100 (Suffolk Avenue), where NY 111 leaves Islip Avenue and changes names to Joshua's Path.

In the middle of the name change, Caleb's Path forks off to the northwest, while NY 111 forks to the northeast along Joshua's Path through a long residential section of Central Islip. Several blocks to the north, NY 111 intersects with CR 67 (Long Island Motor Parkway) at-grade. Now out of Central Islip, the route becomes industrial, crossing into a large merge with CR 17 (Wheeler Road). Adopting the name Wheeler Road, NY 111 enters an interchange with exit 56 of the Long Island Expressway (I-495). Still in the town of Islip, the route bends northwest along Wheeler Road, crossing through town as a two-lane road. Passing the American Real Estate School, the route bends to the northeast, paralleling the Nissequogue River. Passing a campus of Empire State College, NY 111 intersects at-grade with NY 454 (Veterans Highway). At this junction, the route changes names to Hauppague Road, crossing into Smithtown. The route soon intersects with and has a short concurrency with CR 76 (Townline Road).

After forking from CR 76, NY 111 continues northeast along Hauppague Road. On this northeastern stretch, the route crosses at-grade with NY 347 (Smithtown Bypass). Now a two-lane residential street, NY 111 continues northeast, crossing into Village of the Branch. In the Village of the Branch, NY 111 bends northward, crossing into a large commercial area, where it intersects with NY 25 and NY 25A (East Main Street). At this junction, NY 25 continues eastward as Middle Country Road, NY 111 terminates, and NY 25A turns northward on NY 111's right-of-way, changing names to North Country Road.

The origins of NY 111 date back to 1914 when the state of New York awarded a contract on July 29 to improve a 9.56-mile (15.39 km) highway in the towns of Islip and Smithtown to state highway standards. The road covered by the project began at Main Street in East Islip and headed north on Carleton Avenue and Wheeler Road to Central Islip, from where it continued northeast to Village of the Branch via Hauppauge Road. Reconstruction of the road cost $263,359 (equivalent to $3.66 million in 2013), and the rebuilt road was added to the state highway system on June 1, 1922, as unsigned State Highway 1208 (SH 1208). In the 1930 renumbering of state highways in New York, hundreds of state-maintained highways were assigned a posted route designation for the first time. One of these was SH 1208, which became NY 111.

On September 13, 1966, NY 111 was realigned south of Central Islip to follow a slightly more westerly alignment to the hamlet of Islip along Joshua's Path and Islip Avenue, two streets that ran parallel to the route's original path on Carleton Avenue and Wheeler Road. Ownership and maintenance of NY 111's former routing was transferred to Suffolk County, which redesignated the road as CR 17. The ultimately cancelled eastern extension of the Northern State Parkway would have crossed NY 111 just south of Village of the Branch at a point north of Mount Pleasant Road. An interchange between the parkway and NY 111 was planned at that location.

The entire route is in Suffolk County.