Rockland County is the southernmost county on the west side of the Hudson River in the U.S. state of New York. It is part of the New York metropolitan area. As of the 2020 U.S. census, the county's population is 338,329, making it the state's third-most densely populated county outside New York City after Nassau and neighboring Westchester counties. The county seat and largest hamlet is New City. Rockland County is accessible via both the New York State Thruway, which crosses the Hudson River to Westchester via the Tappan Zee Bridge over the Tappan Zee, ten exits up from the NYC border; and the Palisades Parkway, four exits up, via the George Washington Bridge. The county's name derives from "rocky land", as the area has been aptly described, largely due to the Hudson River Palisades. The county is part of the Hudson Valley region of the state.

Rockland County is the smallest county by area in New York outside New York City. It comprises five towns, eighteen incorporated villages, eighteen census-designated places, eight hamlets and eleven defunct communities. Rockland County is designated as a Preserve America Community, and nearly a third of the county's area is parkland. The county has the largest Jewish population percentage of any U.S. county, at 31.4%, or 90,000 residents. Rockland also ranked 80th on the list of highest-income counties by median household income in the United States, with a median household income of $82,534 according to the 2010 census.

The area that became Rockland County was originally inhabited by Algonquian-speaking Native Americans, including Munsees and Lenape. The Tappan tribe had a particularly noteworthy presence in the area, extending from present-day Nyack, south to Sparkill and Tappan, down the Hackensack River valley through present-day Bergen County, New Jersey, and also along the Palisades and Hudson River shore all the way to present-day Edgewater, New Jersey.

In 1609, Henry Hudson was the first major English explorer to arrive in the area. Hudson, thinking he had found the legendary "Northwest Passage", sailed on the Half Moon up the river that would one day bear his name, and sailing through present-day Haverstraw before exploring north in present-day Albany.

In the years before 1664 when the area was formally a Dutch colony called New Netherland, present-day Rockland did not have formal European settlements. However, individuals did explore the area and made transactions with Tappan tribe for land with the idea that it could have future use. For example, in 1640, Dutch Captain David Pietersz. de Vries purchased from natives the area where the Sparkill Creek flows into the Hudson River.

In 1664, the British Crown assumed control of New Netherland from the Dutch. In June 1664, the Berkeley-Carteret land grant established the colony of New Jersey, dividing present-day Rockland and Bergen Counties into separate political areas. The northern border of New Jersey was placed in a straight line from the Delaware River at present-day Port Jervis to the Hudson River at 41 degrees even North latitude, where the Palisades Cliffs pause and give way to Sneden's Landing in Orangetown. The state line remains there to this day, though various disagreements along the exact border were had over the years.

In the 1670s, permanent Dutch settlers began to arrive with land grants, starting with the Tappan area. These settlers were eager to escape "city life", moving from Manhattan to Rockland. A number of unique, Dutch-style red sandstone houses still stand, and many place names in the county reveal their Dutch origin.

In 1683, when the Duke of York (who became King James II of England) established the first 12 counties of New York, present-day Rockland County was part of Orange County, known then as "Orange County South of the Mountains". Orangetown was created at the same time under a royal grant, originally encompassing all of modern Rockland County. Around this time, as the English began to colonize Nyack and Tappan, the Native Americans began to leave Rockland in search of undisturbed land further north.