Collegiate School is an all-boys private school on the Upper West Side of Manhattan, New York City. Founded by Dutch colonists in either 1628 or 1638, it is the nation's oldest private secondary school, and claims to be the nation's oldest school without qualification. It educates around 670 boys in grades K–12, with approximately 50–55 students per grade.

It is a member of both the New York Interschool Association and the Ivy Preparatory School League, two groups of New York City secondary schools.

In 1628, the Reverend Jonas Michaëlius, the first minister of the Dutch Reformed Church in America, arrived in the Dutch colony of New Netherland. That August, he wrote to an Amsterdam preacher that he wanted "to place [Native American] students under the instruction of some experienced and godly schoolmaster, where they may be instructed not only to speak, read, and write in our language, but also especially in the fundamentals of our Christian religion." It appears that some form of Dutch "school" existed on Manhattan no later than 1632, when a marriage contract instructed a husband to "keep [his step-children] at school."

In August 1637, the Dutch West India Company and the Classis of Amsterdam, the supervising body of the Dutch Reformed Church, licensed Adam Roelantsen to open a state-funded school in Manhattan, which opened in 1638. The school began as a co-ed school located south of Canal Street,^{[citation needed]} but became an all-boys institution at the end of the 19th century.

The school has revised its founding date over the years. The school initially set the founding date at 1633, but later changed it to 1638, as archival research suggested that Roelantsen visited Manhattan in 1633, but was not teaching at that time. In 1982, the New-York Historical Society revealed a 1628 letter from Michaëlius discussing his teaching efforts. Based on this letter, the school announced in 1984 that it was changing the foundation date to 1628, allowing it to (somewhat controversially) claim the title of the oldest school in America. (Boston Latin School, the nation's oldest public school, was founded in 1635, and Harvard University, the nation's oldest university, was founded in 1636.)

Collegiate asserts continuity with Michaëlius' tutoring work because "the minister's efforts later led to the founding of Collegiate" and Dutch Reformed ministers frequently took on teaching duties in the Dutch colonies. In 2009, a Dutch historian contested Collegiate's position and supported the 1638 date, albeit in a summary footnote.

Collegiate traditionally catered to Manhattan's Dutch settler population, even after the English conquest. Under English and later British rule, in the years before free, universal public education, the various religious denominations sponsored schools for their own communities. Collegiate was funded by New York's Collegiate Reformed Protestant Dutch Church, hence the name. (The term "collegiate" refers to a network of churches that "shared ministers and kept a single set of registers.")

The school stubbornly retained its Dutch identity for over a century after the English conquest. In 1705, it fended off the English colonial governor's (Lord Cornbury) attempts to take over the school, after which Cornbury encouraged William Huddleston to establish the Anglican Trinity School in 1709. Classes were conducted exclusively in Dutch until 1773. The school educated only members of the Dutch Reformed Church until 1869, and even after 1869, Dutch Reformed children had an advantage in the admissions process.