Rensselaer County is a county in the U.S. state of New York. As of the 2010 census, the population was 159,429. Its name is in honor of the family of Kiliaen van Rensselaer, the original Dutch owner of the land in the area. Its county seat is Troy. It is part of the Albany-Schenectady-Troy Metropolitan Statistical Area.
The area that is now Rensselaer County was inhabited by the Algonquian-speaking Mohican Indian tribe at the time of European encounter. Kiliaen van Rensselaer, a Dutch jeweler and merchant, purchased the area in 1630 and incorporated it in his patroonship Rensselaerswyck. (It was part of the Dutch colony New Netherland).
The land passed into English rule in 1664; the Dutch regained control in 1673, but the English took it back in 1674. Until 1776, the year of American independence, the county was under English or British control. The county was not organized as a legal entity until after the Revolution in 1791, when it was created from an area that was originally part of the very large Albany County.
In 1807, in a county re-organization, the rural sections of Troy were set off as Towns, and the city was incorporated. The two towns created were Brunswick and Grafton, both named after British dukes, (the of Brunswick and Grafton). A third town, Philipstown, was set off in 1806. In 1808 it was renamed Nassau after the duke of that area.