Cayey (Spanish pronunciation: [kaˈʝej]), officially Cayey de Muesas, is a mountain town and municipality in central Puerto Rico located on the Sierra de Cayey within the Central Mountain range, north east of Salinas and north of Guayama; south of Cidra and Caguas; east of Aibonito and west of San Lorenzo. Cayey is spread over 21 barrios plus Cayey Pueblo (the downtown area and the administrative center). It is part of the San Juan-Caguas-Guaynabo Metropolitan Statistical Area.

Cayey is notable for its surrounding mountains. The city has been actively growing since the 1990s, evidenced by its designation as a Metropolitan Area by the U.S. Census Bureau. It has experienced significant growth in commerce, and many major retailers, such as Wal-Mart have opened stores in the city. Industries in Cayey include sugar, tobacco and poultry. For tobacco there is a well-known company called Consolidated Cigar Corp. A new coliseum and hospital facilities have also been built. Coca-Cola is a major corporation that has a manufacturing facility in the town. Cayey is also host to one of the campuses of the University of Puerto Rico, the University of Puerto Rico at Cayey.

Cayey was founded on August 17, 1773, by Juan Mata Vázquez, who became its first mayor. It is popularly said that Cayey derives its name from the Taíno word for "a place of waters". It was named Cayey de Muesas in honor of Miguel de Muesas, the then governor of Puerto Rico. The town is located in a valley nestled between Puerto Rico's Cordillera Central mountain range and the Sierra de Cayey at roughly the midpoint of routes PR-1 and PR-52. The routes lead to a popular drive that provides panoramic views of the island from its highest points.

Puerto Rico was ceded by Spain in the aftermath of the Spanish–American War under the terms of the Treaty of Paris of 1898 and became a territory of the United States. In 1899, the United States Department of War conducted a census of Puerto Rico finding that the population of Cayey was 14,442.

An education base began in the early to mid-1950s when the Interamerican University opened a branch in Cayey providing teacher training through a night class scheme. In 1967 the University of Puerto Rico opened a campus in the former Henry Barracks Military Reservation, and in the early 1980s El Turabo University, subsidiary of the Ana G. Mendez conglomerate opened a campus in the old tobacco factory at the entrance of town. The Interamerican University will be opening a Graduate campus in front of the main town square (downtown), and there are conversations with a foreign educational concern to open a technology campus using the buildings left over by the Gordonshire Knitting Mills. There are three major private schools: Radians School, the long established Colegio de Nuestra Senora de la Merced and La Milagrosa School. Cayey's health care base expanded in the mid-1960s with the Mennonite Medical Center and a Municipal Hospital along with laboratories, and urgent care centers that cater to the poor and the elderly.

With the construction of the Interstate (PR-52), Cayey has evolved into a "bedroom community" with gated housing developments, located just 30 minutes from San Juan and 45 minutes away from Ponce. With pleasant weather and good private schools, Cayey has become a prime location for the affluent.

Hurricane Maria on September 20, 2017, triggered numerous landslides in Cayey with significant amount of rain and wind. The hurricane destroyed 3,000 homes in Cayey.

Henry Barracks Military Reservation was a lifeline for the residents of Cayey from 1901 to 1966, when it was declared excess land and passed to the General Services Administration for decommission. The property consisted of 178.03 hectares (439.92 acres) (). The property was divided into three prominent encampments: the Spanish Camp or Campamento Español (6.1 hectares (15 acres)), Camp Henry or Henry Barracks, the Home of the third Battalion of the 65th Inf. Regiment that consisted of 151 hectares (372 acres), and 27 hectares (67 acres) the Cayey Naval Radio Station (27 hectares (67 acres) )). This reservation is situated in the east of the town of Cayey. The Spanish Reservation containing an area of approximately 6.1 hectares (15 acres), known as Hospital Hill was set apart by Executive Order of June 30, 1903, under an Act of Congress approved on July 7, 1902. The main army post was located in the northern part of the reservation, initially housing the Puerto Rico Volunteer Infantry Regiment.