Monsey () is a hamlet and census-designated place in the town of Ramapo, Rockland County, New York, United States, located north of Airmont; east of Viola; south of New Hempstead; and west of Spring Valley. The village of Kaser is completely surrounded by the hamlet of Monsey. The 2010 census listed the population at 18,412.
The hamlet has a large and growing community of Hasidic Jews.
Rockland County was inhabited by the Munsee band of Lenape Native Americans, who were speakers of the Algonquian languages. Monsey Glen, an Indian encampment, is located west of the intersection of State Route 59 and State Route 306. Numerous artifacts have been found there and some rock shelters are still visible. The Monsey railroad station, which received its name from an alternate spelling of the Munsee Lenape, was built when the New York & Erie Railroad passed through the glen in 1841.
In the 1950s, Monsey was a one stoplight town with a single yeshiva. By 1997, Monsey had 112 synagogues and 45 yeshivas.
Located in Monsey is the Houser-Conklin House, listed on the National Register of Historic Places in 2010.
On December 28, 2019 it was the site of a mass stabbing in the home of a Hasidic rabbi who was hosting a Hanukkah party, leaving five injured.
Monsey is located at 41°7′10″N 74°3′57″W .
According to the United States Census Bureau, the CDP has a total area of 2.2 square miles (5.8 km2), of which, 2.2 square miles (5.7 km2) of it is land and 0.04 square miles (0.1 km2) of it (0.90%) is water.
As of the census of 2017, there were 22,043 people, 3,984 households, and 2,596 families residing in the CDP. The population density was 6,554.3 per square mile (2,533.9/km2).